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HUMAN    PHYSIOLOGY 

TWELFTH   EDITION 


BRUBAKER 


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A  COMPEND 


OF 


HUMAN    PHYSIOLOGY 

ESPECIALLY  ADAPTED  FOR  THE  USE 
OF  MEDICAL  STUDENTS 


BY 

ALBERT   P.    BRUBAKER,   A.M.,  M.D. 

AUTHOR    OF     "A     TEXT-BOOK     OF    PHYSIOLOGY";      PROFESSOR     OF     PHYSIOLOGY    AND 

HYGIENE    IN   THE   JEFFERSON    MEDICAL    COLLEGE;     PROFESSOR    OF    PHYSIOLOGY    IN 

THE  PENNSYLVANIA  COLLEGE    OF    DENTAL  SURGERY;     LECTURER    ON    ANATOMY 

AND    PHYSIOLOGY    IN    THE    DREXEL    INSTITUTE    OF    ART,    SCIENCE,    AND 

INDUSTRY;     FELLOW   OF   THE   COLLEGE   OF   PHYSICIANS 


TWELFTH  EDITION— REVISED   AND  ENLARGED 

With  Illustrations  and  a    Table  of  Physiologic  Constants 


PHILADELPHIA 

P.    BLAKISTON'S   SON    &    CO. 

IOI2    WALNUT    STREET 


Entered  according  to  Act  of  Congress,  in  the  year  1905,  by 

P.  BLAKISTON'S  SON  &  CO. 
In  the  Office  of  the  Librarian  of  Congress,  at  Washington,  D.  C. 


PRESS  OF 

THE  NEW  ERA  PRINTING  COMPANY 
LANCASTER,  PA. 


PREFACE  TO  THE  TWELFTH   EDITION. 


A  twelfth  edition  of  the  Compend  having  been  called  for,  the 
author  has  taken  the  opportunity  to  revise  some  of  the  paragraphs 
and  to  add  some  new  material  which  it  is  hoped  will  increase  its 
usefulness  to  the  student.  While  many  of  the  changes  that  have 
been  made  will  be  found  distributed  throughout  the  body  of  the 
work,  the  principal  changes  will  be  found  in  the  section  relating 
to  the  cranial  nerves.  In  this  section,  the  newer  views  regarding 
the  organ  and  relation  of  the  various  cranial  nerves  are  briefly 
presented. 

That  the  Compend  may  continue  to  aid  the  student  for  whom 
it  was  primarily  written  is  the  wish  of 

ALBERT  P.  BRUBAKER. 
November   i,   1905. 


CONTENTS. 


INTODUCTION    , i 

GENERAL  STRUCTURE  OF  THE  ANIMAL  BODY 3 

CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY 8 

PHYSIOLOGY  OF  THE  CELL 29 

HISTOLOGY  OF  THE  EPITHELIAL  AND  CONNECTIVE  TISSUES 35 

MECHANISM   OF   THE   SKELETON 43 

GENERAL  PHYSIOLOGY  OF  MUSCULAR  TISSUE }8 

SPECIAL  PHYSIOLOGY  OF  MUSCLES 61 

PHYSIOLOGY   OF   NERVE  TISSUE 68 

FOODS  AND  DIETETICS 86 

DIGESTION    96 

ABSORPTION    114 

BLOOD    123 

CIRCULATION    OF    THE    BLOOD 129 

RESPIRATION    1 42 

ANIMAL  HEAT   150 

SECRETION    153 

Mammary    Glands 155 

Vascular  Glands 158 

EXCRETION   164 

Kidneys    164 

Liver 162 

Skin     177 

THE  CENTRAL  ORGANS  OF  THE  NERVE  SYSTEM 180 

SPINAL  CORD 1 82 

MEDULLA  OBLONGATA 193 

PONS  VAROLII   197 

CRURA  CEREBRI   197 

CORPORA  QUADRIGEMINA   198 

CORPORA  STRIATA  AND  OPTIC  THALAMI 199 

CEREBELLUM 201 

CEREBRUM 201 

SYMPATHETIC  NERVOUS  SYSTEM 215 

vii 


Vll]  CONTENTS. 

PAGE 

CRANIAL  NERVES 218 

SENSE  OF  TOUCH    233 

SENSE  OF  TASTE ^ 234 

SENSE  OF  SMELL 236 

SENSE  OF  SIGHT 237 

SENSE  OF  HEARING 249 

VOICE  AND   SPEECH 258 

REPRODUCTION    • 261 

Generative  Organs  of  the  Female 261 

Generative  Organs  of  the  Male 264 

Development  of  Accessory  Structures 266 

Development  of  the  Embryo 271 

TABLE  OF  PHYSIOLOGIC  CONSTANTS 278 

TABLE  SHOWING  RELATION  OF  WEIGHTS  AND  MEASURES 281 

INDEX    283 


A  COMPEND 


OF 


HUMAN  PHYSIOLOGY. 


Introduction. — An  animal  organism  in  the  living  condition  exhibits 
a  series  of  phenomena  which  relate  to  growth,  movement,  mentality, 
and  reproduction.  During  the  period  preceding  birth,  as  well  as  dur- 
ing the  period  included  between  birth  and  adult  life,  the  individual 
grows  in  size  and  complexity  from  the  introduction  and  assimilation 
of  material  from  without.  Throughout  its  life  the  animal  exhibits  a 
series  of  movements,  in  virtue  of  which  it  not  only  changes  the  rela- 
tion of  one  part  of  its  body  to  another,  but  also  changes  its  position 
in  space.  If,  in  the  execution  of  these  movements,  the  parts  are 
directed  to  the  overcoming  of  opposing  forces,  such  as  gravity,  fric- 
tion, cohesion,  elasticity,  etc.,  the  animal  may  be  said  to  be  doing 
work.  The  result  of  normal  growth  is  the  attainment  of  a  physical 
development  that  will  enable  the  animal,  and,  more  especially,  man,  to 
perform  the  work  necessitated  by  the  nature  of  its  environment  and 
the  character  of  its  organization.  In  man,  and  probably  in  lower 
animals  as  well,  mentality  manifests  itself  as  intellect,  feeling,  and 
volition.  At  a  definite  period  in  the  life  of  the  animal  it  reproduces 
itself,  in  consequence  of  which  the  species  to  which  it  belon'gs  is 
perpetuated. 

The  study  of  the  phenomena  of  growth,  movement,  mentality,  and 
reproduction  constitutes  the  science  of  ANIMAL  PHYSIOLOGY.  But  as 
these  general  activities  are  the  resultant  of  and  dependent  on  the 
special  activities  of  the  individual  structures  of  which  an  animal  body 
is  composed,  Physiology  in  its  more  restricted  "and  generally  accepted 
sense  is  the  science  which  investigates  the  actions  or  functions  of 
the  individual  organs  and  tissues  of  the  body  and  the  physical  and 
chemic  conditions  which  underlie  and  determine  them. 
2  1 


Z  HUMAN    PHYSIOLOGY. 

This  may  naturally  be  divided  into  : 

1.  Special  physiology,  the  object  of  which  is  a  study  of  the  vital  phe- 
nomena   or   functions    exhibited   by   the   organs    of   any   individual 
animal. 

2.  Comparative  physiology,  the  object  of  which  is  a  comparison  of  the 
vital  phenomena  or  functions   exhibited  by  the   organs   of  two   or 
more  animals,  with  a  view  to  unfolding  their  points  of  resemblance 
or  dissimilarity. 

Human  physiology  is  that  department  of  physiologic  science  which 
has  for  its  object  the  study  of  the  functions  of  the  organs  of  the 
human  body  in  a  state  of  health. 

Inasmuch  as  the  study  of  function,  or  physiology,  is  associated  with 
and  dependent  on  a  knowledge  of  structure,  or  anatomy,  it  is  essential 
that  the  student  should  have  a  general  acquaintance  not  only  with  the 
structure  of  man,  but  with  that  of  typical  forms  of  lower  animal 
life  as  well. 

If  the  body  of  any  animal  be  dissected,  it  will  be  found  to  be  com- 
posed of  a  number  of  well-defined  structures,  such  as  heart,  lungs, 
stomach,  brain,  eye,  etc.,  to  which  the  term  organ  was  originally 
applied,  for  the  reason  that  they  were  supposed  to  be  instruments 
capable  of  performing  some  important  act  or  function  in  the  general 
activities  of  the  body.  Though  the  term  organ  is  usually  employed 
to  designate  the  larger  and  more  familiar  structures  just  mentioned, 
it  is  equally  applicable  to  a  large  number  of  other  structures  which, 
though  possibly  less  obvious,  are  equally  important  in  maintaining 
the  life  of  the  individual — e.  g.,  bones,  muscles,  nerves,  skin,  teeth, 
glands,  blood-vessels,  etc.  Indeed,  any  complexly  organized  structure 
capable  of  performing  some  function  may  be  described  as  an  organ. 
A  description  of  the  various  organs  which  make  up  the  body  of  an 
animal,  their  external  form,  their  internal  arrangement,  their  rela- 
tions to  one  another,  constitutes  the  science  of  ANIMAL  ANATOMY. 

This  may  naturally  be  divided  into  : 

1.  Special  anatomy,  the  object  of  which  is  the  investigation  of  the 
construction,    form,    and   arrangement   of   the   organs   of   any   indi- 
vidual animal. 

2.  Comparative  anatomy,  the  object  of  which  is  a  comparison  of  the 
organs  of  two  or  more  animals,  with  a  view  to  determining  their 
points  of  resemblance  or  dissimilarity. 


GENERAL    STRUCTURE   OF   THE   ANIMAL   BODY.  O 

If  the  organs,  however,  are  subjected  to  a  further  analysis,  they 
can  be  resolved  into  simple  structures,  apparently  homogeneous,  to 
which  the  name  tissue  has  been  given — e.  g.,  epithelial,  connective, 
muscle,  and  nerve  tissue.  When  the  tissues  are  subjected  to  micro- 
scopic analysis,  it  is  found  that  they  are  not  homogeneous  in  struc- 
ture, but  composed  of  still  simpler  elements,  termed  cells  and  fibers. 
The  investigation  of  the  internal  structure  of  the  organs,  the  physical 
properties  and  structure  of  the  tissues,  as  well  as  the  structure  of 
their  component  elements,  the  cells  and  fibers,  constitutes  a  depart- 
ment of  anatomic  science  known  as  HISTOLOGY,  or  as  it  is  prosecuted 
largely  with  the  microscope,  MICROSCOPIC  ANATOMY. 

Human  anatomy  is  that  department  of  anatomic  science  which  has 
for  its  object  the  investigation  of  the  construction  of  the  human  body. 


GENERAL  STRUCTURE  OF  THE  ANIMAL  BODY. 

The  body  of  every  animal,  from  fish  to  man,  may  be  divided  into — 

1.  An  axial  and 

2.  An  appendicular  portion.     The  axial  portion  consists  of  the  head, 
neck,  and  trunk  ;  the  appendicular  portion  consists  of  the  anterior 
and   posterior   limbs   or   extremities. 

The  axial  portion  of  all  mammals,  to  which  class  man  zoologically 
belongs,  as  well  as  of  all  birds,  reptiles,  amphibians,  and  osseous  fish, 
is  characterized  by  the  presence  of  a  bony,  segmented  axis,  which  ex- 
tends in  a  longitudinal  direction  from  before  backward,  and  which  is 
known  as  the  vertebral  column  or  backbone.  In  virtue  of  the  exist- 
ence of  this  column  all  the  class  of  animals  just  mentioned  form 
one  great  division  of  the  animal  kingdom,  the  Vertebrata. 

Each  segment,  or  vertebra,  of  this  axis  consists  of — 

1.  A  solid  portion,  known  as  the  body  or  centrum,  and 

2.  A  bony  arch  arising  from  the  dorsal  aspect  and  surmounted  by  a 
spine-like  process. 

At  the  anterior  extremity  of  the  body  of  the  animal  the  vertebrae 
are  variously  modified  and  expanded,  and,  with  the  addition  of  new 
elements,  form  the  skull ;  at  the  posterior  extremity  they  rapidly 
diminish  in  size,  and  terminate  in  man  in  a  short,  tail-like  process. 
In  many  animals,  however,  the  vertebral  column  extends  for  a  con- 
siderable distance  beyond  the  trunk  into  the  tail.  The  vertebral 
column  may  be  regarded  as  the  foundation  element  in  the  plan  of 


HUMAN   PHYSIOLOGY. 


FIG.  i. — DIAGRAMMATIC  LONGITUDINAL 
SECTION  OF  THE  BODY. 

V,V.  Bodies  of  the  vertebrae  which  divide 
the  body  into  the  dorsal  and  ventral 
cavities,  a,  a'.  The  dorsal  cavity.  C,  pf. 
The  abdominal  and  thoracic  divisions 
of  the  ventral  cavity,  separated  from 
each  other  by  a  transverse  muscular 
partition,  the  diaphragm  d.  B.  The 
brain.  Sp.  C.  The  spinal  cord.  e.  The 
esophagus.  S.  The  stomach,  from 
which  continues  the  intestine  to  the 
opening  of  the  posterior  portion  of  the 
body.  /.  The  liver,  p.  The  pancreas. 
k.  The  kidney,  o.  The  bladder.  /'. 
The  lungs,  h.  The  heart. 


organization  of  all  the  higher 
animals  and  the  center  around 
which  the  rest  of  the  body  is 
developed  and  arranged  with  a 
certain  degree  of  conformity. 
In  all  vertebrate  animals  the 
bodies  of  the  segments  of  the 
vertebral  column  form  a  par- 
tition which  serves  to  divide 
the  trunk  of  the  body  into  two 
cavities — viz.,  the  dorsal  and 
the  ventral. 

The  dorsal  cavity  is  found 
in  the  head.  Its  walls  are 
not  only  in  the  trunk,  but  also 
formed  partly  by  the  arches 
which  arise  from  the  posterior 
or  dorsal  surface  of  the  ver- 
tebrae and  partly  by  the  bones 
of  the  skull.  If  a  longitudinal 
section  be  made  through  the 
center  of  the  vertebral  column, 
and  including  the  head,  the  dor- 
sal cavity  will  be  observed  run- 
ning through  its  entire  extent. 
(See  Fig.  i.)  Though  for  the 
most  part  it  is  quite  narrow,  at 
the  anterior  extremity  it  is  en- 
larged and  forms  the  cavity  of 
the  skull.  This  cavity  is  lined 
by  a  membranous  canal,  the 
neural  canal,  in  which  is  con- 
tained the  brain  and  the  neural 
or  spinal  cord.  Through  open- 
ings in  the  sides  of  the  dorsal 
cavity  nerves  pass  out  which 
connect  the  brain  and  spinal 
cord  with  all  the  structures  of 
the  body. 


GENERAL   STRUCTURE  OF  THE  ANIMAL  BODY. 

The  ventral  cavity  is  confined  mainly  to  the  trunk  of  the  body. 
Its  walls  are  formed  by  muscles  and  skin,  strengthened  in  most 
animals  by  bony  arches,  the  ribs.  Within  the  ventral  cavity  is 
contained  a  musculo-membranous  tube  or  canal  known  as  the  ali- 
mentary or  food  canal,  which  begins  at  the  mouth  on  the  ventral 
side  of  the  head,  and,  after  passing  through  the  neck  and  trunk,  ter- 
minates at  the  posterior  extremity  of  the  trunk  at  the  anus.  It 
may  be  divided  into  mouth,  pharynx,  esophagus,  stomach,  small  and 
large  intestines. 

In  all  mammals  the  ventral  cavity  is  divided  by  a  musculo-mem- 
branous partition  into  two  smaller  cavities,  the  thorax  and  abdomen. 
The  former  contains  the  lungs,  heart  and  its  great  blood-vessels,  and 
the  anterior  part  of  the  alimentary  canal,  the  gullet  or  esophagus ; 
the  latter  contains  the  continuation  of  the  alimentary  canal — that  is, 
the  stomach  and  intestines — and  the  glands  in  connection  with  it,  the 
liver  and  pancreas.  In  the  posterior  portion  of  the  abdominal  cavity 
are  found  the  kidneys,  ureters,  and  bladder,  and  in  the  female  the  or- 
gans of  reproduction.  The  thoracic  and  abdominal  cavities  are  each  lined 
by  a  thin  serous  membrane,  known,  respectively,  as  the  pleural  and 
peritoneal  membranes,  which,  in  addition,  are  reflected  over  the  sur- 
faces of  the  organs  contained  within  them.  The  alimentary  canal 
and  the  various  cavities  connected  with  it  are  lined  throughout  by  a 
mucous  membrane.  The  surface  of  the  body  is  covered  by  the  skin. 
This  is  composed  of  an  inner  portion,  the  derma,  and  an  outer  portion, 
the  epidermis.  The  former  consists  of  fibers,  blood-vessels,  nerves, 
etc. ;  the  latter  of  layers  of  scales  or  cells.  Embedded  within  the 
skin  are  numbers  of  glands,  which  exude,  in  the  different  classes  of 
animals,  sweat,  oily  matter,  etc.  Projecting  from  the  surface  of  the 
skin  are  hairs,  bristles,  feathers,  claws.  Beneath  the  skin  are  found 
muscles,  bones,  blood-vessels,  nerves,  etc. 

The  appendicular  portion  of  the  body  consists  of  two  pairs  of 
symmetric  limbs,  which  project  from  the  sides  of  the  trunk,  and 
which  bear  a  determinate  relation  to  the  vertebral  column.  They 
consist  fundamentally  of  bones,  surrounded  by  muscles,  blood-vessels, 
nerves  and  lymphatics.  The  limbs*  though  having  a  common  plan 
of  organization,  are  modified  in  form  and  adapted  for  prehension 
and  locomotion  in  accordance  with  the  needs  of  the  animal. 

Anatomic  Systems. — All  the  organs  of  the  body  which  have  cer- 
tain peculiarities  of  structure  in  common  are  classified  by  anatomists 


HUMAN    PHYSIOLOGY. 

into  systems — e.  g.}  the  bones,  collectively,  constitute  the  bony  or 
osseous  system ;  the  muscles,  the  nerves,  the  skin,  constitute,  respec- 
tively, the  muscular,  the  nervous,  and  the  tegumentary  systems. 

Physiologic  Apparatus. — More  important  from  a  physiologic  point 
of  view  than  a  classification  of  organs  based  on  similarities  of  struc- 
ture is  the  natural  association  of  two  or  more  organs  acting  together 
for  the  accomplishment  of  some  definite  object,  and  to  which  the 
term  physiologic  apparatus  has  been  applied.  While  in  the  com- 
munity of  organs  which  together  constitute  the  animal  body  each  one 
performs  some  definite  function,  and  the  harmonious  cooperation  of 
all  is  necessary  to  the  life  of  the  individual,  everywhere  it  is  found 
that  two  or  more  organs,  though  performing  totally  distinct  func- 
tions, are  cooperating  for  the  accomplishment  of  some  larger  or 
compound  function  in  which  their  individual  functions  are  blended — 
e.  g.,  the  mouth,  stomach,  and  intestines,  with  the  glands  connected 
with  them,  constitute  the  digestive  apparatus,  the  object  or  function 
of  which  is  the  complete  digestion  of  the  food.  The  capillary  blood- 
vessels and  lymphatic  vessels  of  the  body,  and  especially  those  in 
relation  to  the  villi  of  the  small  intestine,  constitute  the  absorptive 
apparatus,  the  function  of  which  is  the  introduction  of  new  material 
into  the  blood.  The  heart  and  blood-vessels  constitute  the  circulatory 
apparatus,  the  function  of  which  is  the  distribution  of  blood  to  all 
portions  of  the  body.  The  lungs  and  trachea,  together  with  the 
diaphragm  and  the  walls  of  the  chest,  constitute  the  respiratory  ap- 
paratus, the  function  of  which  is  the  introduction  of  oxygen  into 
the  blood  and  the  elimination  from  it  of  carbon  dioxid  and  other 
injurious  products.  The  kidneys,  the  ureters,  and  the  bladder  con- 
stitute the  urinary  apparatus.  The  skin,  with  its  sweat-glands,  con- 
stitutes the  perspiratory  apparatus,  the  functions  of  both  being  the 
excretion  of  waste  products  from  the  body.  The  liver,  the  pancreas, 
the  mammary  glands,  as  well  as  other  glands,  each  form  a  secretory 
apparatus  which  elaborates  some  specific  material  necessary  to  the 
nutrition  of  the  individual.  The  functions  of  these  different  physi- 
ologic apparatus — e.  g.,  digestion,  absorption  of  food,  elaboration  of 
blood,  circulation  of  blood,  respiration,  production  of  heat,  secretion, 
and  excretion — are  classified  as  nutritive  functions,  and  have  for 
their  final  object  the  preservation  of  the  individual. 

The  nerves  and  muscles  constitute  the  nervo-muscular  apparatus, 
the  function  of  which  is  the  production  of  motion.  The  eye,  the 


GENERAL   STRUCTURE  OF   THE  ANIMAL   BODY.  / 

ear,  the  nose,  the  tongue,  and  the  skin,  with  their  related  structures, 
constitute,  respectively,  the  visual,  auditory,  olfactory,  gustatory,  and 
tactile  apparatus,  the  function  of  which,  as  a  whole,  is  the  reception  of 
impressions  and  the  transmission  of  nerve  impulses  to  the  brain, 
where  they  give  rise  to  visual,  auditory,  olfactory,  gustatory,  and 
tactile  sensations. 

The  brain,  in  association  with  the  sense  organs,  forms  an  apparatus 
related  to  mental  processes.  The  larynx  and  its  accessory  organs — 
the  lungs,  trachea,  respiratory  muscles,  the  mouth  and  resonant 
cavities  of  the  face — form  the  vocal  and  articulating  apparatus,  by 
means  of  which  voice  and  articulate  speech  are  produced.  The 
functions  exhibited  by  the  apparatus  just  mentioned — viz.,  motion, 
sensation,  language,  mental  and  moral  manifestations — are  classified 
as  functions  of  relation,  as  they  serve  to  bring  the  individual  into 
conscious  relationship  with  the  external  world. 

The  ovaries  and  the  testes  are  the  essential  reproductive  organs, 
the  former  producing  the  germ-cell,  the  latter  the  spermatic  element ; 
together  with  their  related  structures, — the  fallopian  tubes,  uterus, 
and  vagina  in  the  female,  and  the  urogenital  canal  in  the  male, — 
they  constitute  the  reproductive  apparatus  characteristic  of  the  two 
sexes.  Their  cooperation  results  in  the  union  of  the  germ-cell  and 
spermatic  element  and  the  consequent  development  of  a  new  being. 
The  function  of  reproduction  serves  to  perpetuate  the  species  to 
which  the  individual  belongs. 

The  animal  body  is  therefore  not  a  homogeneous  organism,  but  one 
composed  of  a  large  number  of  widely  dissimilar  but  related  organs. 
But  as  all  vertebrate  animals  have  the  same  general  plan  of  organiza- 
tion, there  is  a  marked  similarity  both  in  form  and  structure  among 
corresponding  parts  of  different  animals.  Hence  it  is  that  in  the 
study  of  human  anatomy  a  knowledge  of  the  form,  construction,  and 
arrangement  of  the  organs  in  different  types  of  animal  life  is  essential 
to  its  correct  interpretation  ;  also  it  is  that  in  the  investigation  and 
comprehension  of  the  complex  problems  of  human  physiology  a 
knowledge  of  the  functions  of  the  organs  as  they  manifest  them- 
selves in  the  different  types  of  animal  life  is  indispensable.  As  many 
of  the  functions  of  the  human  body  are  not  only  complex,  but  the 
organs  exhibiting  them  are  practically  inaccessible  to  investigation, 
we  must  supplement  our  knowledge  and  judge  of  their  functions  by 
analogy,  by  attributing  to  them,  within  certain  limits,  the  functions 
revealed  by  experimentation  upon  the  corresponding  but  simpler 


HUMAN   PHYSIOLOGY. 

organs  of  lower  animals.  This  experimental  knowledge  corrected 
by  a  study  of  the  clinical  phenomena  of  disease  and  the  results  of 
post-mortem  investigations,  forms  the  basis  of  modern  human 
physiology. 


CHEMIC     COMPOSITION     OF     THE     HUMAN 
BODY. 

Since  it  has  been  demonstrated  that  every  exhibition  of  functional 
activity  is  associated  with  changes  of  structure,  it  has  been  apparent 
that  a  knowledge  of  the  chemic  composition  of  the  body,  not  only 
when  in  a  state  of  rest,  but  to  a  far  greater  degree  when  in  a  state 
of  activity,  is  necessary  to  a  correct  understanding  of  the  intimate 
nature  of  physiologic  processes  Though  the  analysis  of  the  dead 
body  is  comparatively  easy,  the  determination  of  the  successive 
changes  in  composition  of  the  living  body  is  attended  with  many 
difficulties.  The  living  material,  the  bioplasm,  is  not  only  complex 
and  unstable  in  composition,  but  extremely  sensitive  to  all  physical 
and  chemic  influences.  The  methods,  therefore,  which  are  em- 
ployed for  analysis  destroy  its  composition  and  vitality,  and  the 
products  which  are  obtained  are  peculiar  to  dead  rather  than  to 
living  material. 

Chemic  analysis,  therefore,  may  be  directed — 

1.  To  the  determination  of  the  composition  of  the  dead  body. 

2.  To   the    determination   of   the   successive    changes    in   composition 
which  the  living  bioplasm  undergoes  during  functional  activity. 

A  chemic  analysis  of  the  dead  body,  with  a  view  to  disclosing 
the  substances  of  which  it  is  composed,  their  properties,  their  inti- 
mate structure,  their  relationship  to  one  another,  constitutes  what 
might  be  termed  CHEMIC  ANATOMY.  An  investigation  of  the  living 
material  and  of  the  successive  changes  it  undergoes  in  the  per- 
formance of  its  functions  constitutes  what  has  been  termed  CHEMIC 
PHYSIOLOGY  or  PHYSIOLOGIC  CHEMISTRY. 

By  chemic  analysis  the  animal  body  can  be  reduced  to  a  number 
of  liquid  and  solid  compounds  which  belong  to  both  the  inorganic 
and  organic  worlds.  These  compounds,  resulting  from  a  proximate 
analysis,  have  been  termed  proximate  principles.  That  they  may 
merit  this  term,  however,  they  must  be  obtained  in  the  form  under 


CHEMIC   COMPOSITION   OF  THE   HUMAN   BODY.  9 

which  they  exist  in  the  living  condition.  The  organic  compounds 
consist  of  representatives  of  the  carbohydrate,  fatty,  and  proteid 
groups  of  organic  bodies  ;  the  inorganic  compounds  consist  of  water, 
various  acids,  and  inorganic  salts. 

The  compounds  or  proximate  principles  thus  obtained  can  be  further 
resolved  by  an  ultimate  analysis  into  a  small  number  of  chemic  ele- 
ments which  are  identical  with  elements  found  in  many  other  organic 
as  well  as  inorganic  compounds.  The  different  chemic  elements 
which  are  thus  obtained,  and  the  percentage  in  which  they  exist  in 
the  body,  are  as  follows — viz.,  oxygen,  72  per  cent.;  hydrogen,  9.10; 
nitrogen,  2.5;  carbon,  13.50;  phosphorus,  1.15;  calcium,  1.30;  sul- 
phur, 0.147;  sodium,  o.io;  potassium,  0.026;  chlorin,  0.085;  fluorin, 
iron  silicon,  magnesium,  in  small  and  variable  amounts. 

THE  CARBOHYDRATES. 

The  carbohydrates  constitute  a  group  of  organic  bodies,  consisting 
mainly  of  starches  and  sugars,  having  their  origin  for  the  most  part 
in  the  vegetable  world.  In  many  respects  they  are  closely  related, 
and  by  appropriate  means  are  readily  converted  into  one  another. 
In  composition  they  consist  of  the  elements  carbon,  hydrogen,  and 
oxygen.  As  their  name  implies,  the  hydrogen  and  oxygen  are  present 
in  the  majority  of  these  compounds  in  the  proportion  to  form  water, 
or  as  2:1.  The  molecule  of  the  carbohydrates  just  mentioned  con- 
sists of  either  six  atoms  of  carbon  or  a  multiple  of  six ;  in  the  latter 
case  the  quantity  of  hydrogen  and  oxygen  taken  up  by  the  carbon 
is  increased,  though  the  ratio  remains  unchanged. 

The  carbohydrates  may  be  divided  into  three  groups — viz.:  (i) 
Amyloses,  including  starch,  dextrin,  glycogen,  and  cellulose;  (2) 
dextroses,  including  dextrose,  levulose,  galactose ;  (3)  saccharoses, 
including  saccharose,  lactose,  and  maltose.  According  to  the  number 
of  carbon  atoms  entering  into  the  second  group  (six),  they  are  fre- 
quently termed  monosaccharids  ;  those  of  the  third  group,  disaccha- 
rids — twice  six ;  those  of  the  first  group,  polysaccharids — multiples 
of  six. 

Though  but  few  of  the  members  of  the  carbohydrate  group  are 
constituents  of  the  human  body,  yet  on  account  of  their  importance 
as  foods,  and  their  relation  to  one  another,  a  few  of  their  chemic 
features  will  be  stated  in  this  connection. 


10  HUMAN   PHYSIOLOGY. 

i.  AMYLOSES,  (C6H]005)n. 

Starch  is  widely  distributed  in  the  vegetable  world,  being  abundant 
in  the  seeds  of  the  cereals,  leguminous  plants,  and  in  the  tubers  and 
roots  of  some  vegetables.  It  occurs  in  the  form  of  microscopic 
granules,  which  vary  in  size,  shape,  and  appearance,  according  to 
the  plant  from  which  they  are  obtained.  Each  granule  presents  a 
nucleus,  or  hilum,  around  which  is  arranged  a  series  of  eccentric 
rings,  alternately  light  and  dark.  The  granule  consists  of  an  en- 
velope and  stroma  of  cellulose,  containing  in  its  meshes  the  true 
starch  material — granulose.  Starch  is  insoluble  in  cold  water  and 
alcohol.  When  heated  with  water  up  to  70°  C,  the  granules  swell, 
rupture,  and  liberate  the  granulose,  which  forms  an  apparent  solu- 
tion ;  if  present  in  sufficient  quantity,  it  forms  a  gelatinous  mass 
termed  starch  paste.  On  the  addition  of  iodin,  starch  strikes  a 
characteristic  deep  blue  color ;  the  compound  formed — iodid  of 
starch — is  weak,  and  the  color  disappears  on  heating,  but  reappears 
on  cooling. 

Boiling  starch  with  dilute  sulphuric  acid  (twenty-five  per  cent.) 
converts  it  into  dextrose.  In  the  presence  of  vegetable  diastase  or 
animal  ferments,  starch  is  converted  into  maltose  and  dextrose,  two 
forms  of  sugar. 

Dextrin  is  a  substance  formed  as  an  intermediate  product  in  the 
transformation  of  starch  into  sugar.  There  are  at  least  two  principal 
varieties — ery  thro  dextrin,  which  strikes  a  red  color  with  iodin,  and 
achr  oo  dextrin,  which  is  without  color  when  treated  with  this  reagent. 
In  the  pure  state  dextrin  is  a  yellow-white  powder,  soluble  in  water. 
In  the  presence  of  animal  ferments  erythrodextrin  is  converted  into 
maltose. 

Glycogen  is  a  constituent  of  the  animal  liver,  and,  to  a  slight 
extent,  of  muscles  and  of  tissues  generally.  In  the  tissues  of  the 
embryo  it  is  especially  abundant.  When  obtained  in  a  pure  state  it 
is  an  amorphous,  white  powder.  It  is  soluble  in  water,  forming  an 
opalescent  solution.  With  iodin  it  strikes  a  port-wine  color.  In 
some  respects  it  resembles  starch,  in  others  dextrin.  Like  vegetable 
starch,  glycogen  or  animal  starch  can  be  converted  by  dilute  acids  and 
ferments  into  sugar  (maltose). 

Cellulose  is  the  basis  material  of  the  more  or  less  solid  framework 
of  plants.  It  is  soluble  only  in  an  ammoniacal  solution  of  cupric 


CHEMIC   COMPOSITION   OF  THE   HUMAN   BODY.  11 

oxid,  from  which  it  can  be  precipitated  by  acids.  It  is  an  amorphous 
powder ;  dilute  acids  can  convert  it  into  dextrose. 

2.  DEXTROSES,  C6H12O6. 

Dextrose,  glucose,  or  grape-sugar  is  found  in  grapes,  most  sweet 
fruits,  and  honey,  and  as  a  normal  constituent  of  liver,  blood,  muscles, 
and  other  animal  tissues.  In  the  disease  diabetes  mellitus  it  is 
found  also  in  the  urine. 

When  obtained  from  any  source,  it  is  soluble  in  water  and  in  hot 
alcohol,  from  which  it  crystallizes  in  six-sided  tables  or  prisms.  As 
usually  met  with,  it  is  in  the  form  of  irregular,  warty  masses.  It  is 
sweet  to  the  taste  ;  less  so,  however,  than  cane  sugar.  It  is  dextro- 
rotary,  turning  the  plane  of  polarized  light  to  the  right.  In  alkaline 
solutions  dextrose  absorbs  oxygen,  and  hence  in  the  presence  of 
metallic  salts,  copper,  bismuth,  silver,  etc.,  it  acts  as  a  reducing 
agent.  On  this  property  the  various  tests  for  dextrose,  as  well  as 
other  sugars  which  have  the  same  property,  are  based. 

Fehling's  Test. — The  solution  usually  employed  for  both  qualitative 
and  quantitative  purposes  is  a  solution  of  cupric  hydroxid  made  alka- 
line by  an  excess  of  sodium  or  potassium  hydroxid,  with  the  addition 
of  sodium  and  potassium  tartrate.  This  solution,  originally  suggested 
by  Fehling,  bears  his  name.  It  is  made  by  dissolving  cupric  sulphate 
34.64  grams,  potassium  hydroxid  125  grams,  sodium  and  potassium 
tartrate  173  grams  in  i  liter  of  distilled  water. 

The  reaction  is  expressed  by  the  following  equation  : 

CuSO4  +  2KOH  =  Cu(OH)2  +  K2SO4. 

The  object  of  the  sodium  and  potassium  tartrate  is  to  hold  the 
Cu(OH)2  in  solution.  If  a  few  cubic  centimeters  of  this  deep  blue 
solution  be  boiled  and  dextrose  then  added  and  the  solution  again 
heated  to  the  boiling-point,  the  cupric  hydroxid  is  reduced  to  the 
condition  of  a  cuprous  oxid,  which  shows  itself  as  a  red  or  orange- 
yellow  precipitate.  The  color  of  the  precipitate  depends  on  the 
relative  excess  of  either  copper  or  sugar,  being  red  with  the  former, 
orange  or  yellow  with  the  latter.  The  delicacy  of  this  test  is  shown 
by  the  fact  that  a  few  minims  of  this  solution  will  detect  in  one  c.c. 
of  water  the  ^  of  a  milligram  of  sugar. 

For  quantitative  analysis,  ten  c.c.  of  Fehling's  solution,  diluted 
with  forty  c.c.  of  water,  are  heated  in  a  porcelain  capsule,  to  which 
the  dextrose  solution  is  cautiously  added  from  a  buret  until  the  blue 


12  HUMAN   PHYSIOLOGY. 

color  entirely  disappears.  The  strength  of  this  solution  is  such 
that  one  c.c.  is  decolorized  by  five  milligrams  of  sugar,  from  which 
the  percentage  of  sugar  in  any  solution  can  be  determined. 

Fermentation  Test. — If  to  a  solution  of  dextrose  a  small  quantity 
of  the  yeast  plant  be  added,  and  the  solution  kept  at  a  temperature 
of  25°  C.,  it  will  gradually  undergo  fermentation;  that  is,  will  be 
reduced  to  simpler  compounds  and  especially  to  alcohol  and  carbon 
dioxid.  The  change  is  expressed  in  the  following  equation : 

C6H12O6  =  2C2H6O  +  2CO2. 

Dextrose.     Alcohol.       Carbon 

Dioxid. 

About  ninety-five  per  cent,  of  the  dextrose  is  so  changed,  the  remain- 
ing five  per  cent,  yielding  secondary  products — succinic  acid,  glycerin, 
etc. 

Levulose,  or  fruit-sugar,  is  found  in  association  with  dextrose  as  a 
constituent,  of  many  fruits.  It  is  sweeter  than  dextrose  and  more 
soluble  in  both  water  and  dilute  alcohol.  From  alcoholic  solutions 
it  crystallizes  in  fine,  silky  needles,  though  it  usually  occurs  in  the 
form  of  a  syrup. 

Levulose  is  distinguished  from  dextrose  by  its  property  of  turning 
the  plane  of  polarized  light  to  the  left ;  the  extent  to  which  it  does 
so,  however,  varies  with  the  temperature  and  concentration  of  the 
solution. 

Under  the  influence  of  the  yeast  plant  it  slowly  undergoes  fermen- 
tation, yielding  the  same  products  as  dextrose.  It  also  has  a  reduc- 
ing action  on  cupric  oxid. 

Galactose  is  obtained  by  boiling  milk-sugar  (lactose)  with  dilute 
sulphuric  acid.  In  many  chemic  relations  it  resembles  dextrose.  It 
is  less  soluble  in  water,  however,  crystallizes  more  easily,  and  has 
a  greater  dextro-rotary  power.  It  also  undergoes  fermentation  with 
the  yeast  plant. 

3.  SACCHAROSES,  C12H22On. 

Saccharose,  or  cane-sugar,  is  widely  distributed  throughout  the 
vegetable  world,  but  is  especially  abundant  in  sugar-cane,  sorghum 
cane,  sugar-beet,  Indian  corn,  etc.  It  crystallizes  in  large  monoclinic 
prisms.  It  is  soluble  in  water  and  in  dilute  alcohol.  Saccharose  has 
no  reducing  power  on  cupric  oxid,  and  hence  its  presence  can  not  be 


CHEMIC   COMPOSITION   OF  THE   HUMAN   BODY.  13 

detected  by  Fehling's  solution.  It  is  dextro-rotary.  Boiled  with 
dilute  mineral,  as  well  as  organic  acids,  saccharose  combines  with 
water,  and  undergoes  some  change  in  virtue  of  which  it  rotates  the 
plane  of  polarized  light  to  the  left,  and  hence  the  product  is  termed 
invert  sugar.  This  latter  has  been  shown  to  be  a  mixture  of  equal 
quantities  of  levulose  and  dextrose.  This  inversion  of  saccharose 
through  hydration  and  decomposition  is  expressed  by  the  following 
equation : 

C12H220U  +  H20  =  C6H1206  +  C6H1206. 

Saccharose.  Water.      Levulose.       Dextrose. 


Invert  Sugar. 

Saccharose  is  not  directly  fermentable  by  yeast,  but  through  the 
specific  action  of  a  ferment,  invertin  or  invertase,  secreted  by  the 
yeast  plant,  or  the  inverting  ferment  of  the  small  intestine,  it  under- 
goes inversion,  as  previously  stated,  after  which  it  is  readily  fer- 
mented, yielding  alcohol  and  carbon  dioxid. 

Lactose  is  the  form  of  sugar  found  exclusively  in  the  milk  of  the 
mammalia,  from  which  it  can  be  obtained  in  the  form  of  hard, 
white,  rhombic  prisms  united  with  one  molecule  of  water.  It  is 
soluble  in  water,  insoluble  in  alcohol  and  ether.  It  is  dextro-rotary. 
It  requires  cupric  oxid,  but  to  a  less  extent  than  dextrose.  Dilute 
acids  decompose  it  into  equal  quantities  of  dextrose  and  galactose. 
Lactose  is  not  fermentable  with  yeast,  but  in  the  presence  of  the 
lactic  acid  bacillus  it  is  decomposed  into  lactic  acid,  and  finally  into 
butyric  acid,  as  follows : 

C12H22O11  +  H2O  =  4C3H6O3 
Lactose.        Water.   Lactic  Acid. 

2C3H6O3    =     C4H8O2     +     2CO2     +     2H2 
Lactic  Acid.       Butyric  Acid.        Carbon  Free 

Dioxid.         Hydrogen. 

Maltose  is  a  transformation  product  of  starch,  and  arises  whenever 
the  latter  is  acted  on  by  malt  extract  or  the  diastatic  ferments  in 
saliva  and  pancreatic  juice.  It  can  also  be  produced  by  the  action 
of  dilute  sulphuric  acid  on  starch.  The  change  is  expressed  by  the 
following  equation : 

2C6H100B  +  H20  =  C12H22On. 
Starch.         Water.        Maltose. 


14  HUMAN    PHYSIOLOGY. 


Maltose  crystallizes  in  the  form  of  white  needles,  which  are  soluble 
in  water  and  in  dilute  alcohol.  It  is  dextro-rotary.  In  the  presence 
of  ferments  and  dilute  acids  maltose  undergoes  hydration  and  decom- 
position, giving  rise  to  two  molecules  of  dextrose.  It  has  a  reducing 
action  on  cupric  oxid.  Fermentation  is  readily  caused  by  yeast,  but 
whether  directly  or  indirectly  by  inversion  is  somewhat  uncertain. 

Osazones. — All  the  sugars  which  possess  the  power  of  reducing 
cupric  oxid  are  capable  of  combining  with  phenyl-hydrazin,  with  the 
formation  of  compounds  termed  osazones.  The  osazones  so  formed 
are  crystalline  in  structure,  but  have  different  melting  points,  varying 
degrees  of  solubility  and  optic  properties,  all  of  which  serve  to  detect 
the  various  sugars  and  to  distinguish  one  from  the  other.  Of  the 
different  osazones,  phenyl-glucosazone  is  the  most  characteristic,  and 
occurs  in  the  form  of  long,  yellow  needles.  It  may  be  obtained  from 
dextrose  by  the  following  method :  To  fifty  c.c.  of  a  dextrose  solution 
add  2  gm.  of  phenyl-hydrazin  and  two  gm.  of  sodium  acetate,  and 
boil  for  an  hour.  On  cooling,  the  osazone  crystallizes  in  the  form 
of  long,  yellow  needles. 

THE  FATS. 

The  fats  constitute  a  group  of  organic  bodies  found  in  the  tissues 
of  both  vegetables  and  animals.  In  the  vegetable  world  they  are 
largely  found  in  fruits,  seeds,  and  nuts,  where  they  probably  originate 
from  a  transformation  of  the  carbohydrates.  In  the  animal  body  the 
fats  are  found  largely  in  the  subcutaneous  tissue,  in  the  marrow  of 
bones,  in  and  around  various  internal  organs  and  in  milk.  In  these 
situations  fat  is  contained  in  small,  round  or  polygon-shaped  vesicles, 
which  are  united  by  areolar  tissue  and  surrounded  by  blood-vessels. 
At  the  temperature  of  the  body  the  fat  is  liquid,  but  after  death  it 
soon  solidifies  from  the  loss  of  heat. 

The  fats  are  compounds  consisting  of  carbon,  hydrogen,  and  oxy- 
gen, of  which  the  first  is  the  chief  ingredient,  forming  by  weight 
about  seventy-five  per  cent.,  while  the  last  is  present  only  in  small 
quantity.  The  fat,  as  found  in  animals,  is  a  mixture,  in  varying 
proportions  in  different  animals,  of  three  neutral  fats — stearin,  pal- 
mitin,  and  olein.  Each  fat  is  a  derivative  of  glycerin  and  the  par- 
ticular acid  indicated  by  its  name — e.  g.,  stearic  acid,  in  the  case  of 


CHEMIC   COMPOSITION    OF   THE    HUMAN    BODY.  15 

stearin,  etc.     The  reaction  which  takes  place  in  the  combination  of 
glycerin  and  the  acid  is  expressed  in  the  following  equation  : 


C3H5(HO)3  +  (HC1SH3502)3  =  C3H5(C1SH3502)3+  3H20. 
Glycerin.  Stearic  Acid.  Stearin.  Water. 

Hence,  strictly  speaking,  the  fats  are  compound  ethers,  in  which 
the  hydrogen  of  the  organic  acid  is  replaced  by  the  trivalent  radicle, 
tritenyl,  C3H5. 

Stearin,  C3H5(C1S  H^O^,  is  the  chief  constituent  of  the  more  solid 
fats.  It  is  solid  at  ordinary  temperatures,  melting  at  55°  C.,  then 
solidifying  again  as  the  temperature  rises,  until  at  71°  C.  it  melts 
permanently.  It  crystallizes  in  square  tables. 

Palmitin,  C3H5(C1CH31O2)3,  is  a  semifluid  fat,  solid  at  45°  C.  and 
melting  at  62°  C.  It  crystallizes  in  fine  needles,  and  is  soluble  in 
ether. 

Olein,  C3H5(C1SH33O2)3,  is  a  colorless,  transparent  fluid,  liquid  at 
ordinary  temperatures,  only  solidifying  at  o°  C.  It  possesses  marked 
solvent  powers,  and  holds  stearin  and  palmitin  in  solution  at  the  tem- 
perature of  the  body. 

Saponification.  —  When  subjected  to  the  action  of  superheated 
steam,  a  neutral  fat  is  saponified  —  *'.  e.,  decomposed  into  glycerin  and 
the  particular  acid  indicated  by  the  name  of  the  fat  used  :  e.  g., 
stearic,  palmitic,  or  oleic.  The  reaction  is  expressed  as  follows  : 

C3H5(C18H3302)3  +  3H20  =  C3H5(HO)3  +  3(CUHMO2). 
Olein.  Water.  Glycerin.  Oleic  Acid. 

The  fatty  acids  thus  obtained  are  characterized  by  certain  chemic 
features,  as  follows  : 

Stearic  acid  is  a  firm,  white  solid,  fusible  at  69°  C.  It  is  soluble 
in  ether  and  alcohol,  but  not  in  water. 

Palmitic  acid  occurs  in  the  form  of  white,  glistening  scales  or 
needles,  melting  at  62°  C. 

Oleic  acid  is  a  clear,  colorless  liquid,  tasteless  and  odorless  when 
pure.  It  crystallizes  in  white  needles  at  o°  C. 

If  this  saponification  take  place  in  the  presence  of  an  alkali,  —  e.  g., 


16  HUMAN   PHYSIOLOGY. 

potassium  hydroxid,  sodium  hydroxid,  —  the  acid  produced  combines 
at  once  with  the  alkali  to  form  a  salt  known  as  a  soap,  while  the 
glycerin  remains  in  solution.  The  reaction  is  as  follows  : 


3KHO  +  (CuHMOa),  =  3(KC^Hn02)  +  3H2O. 
Potassium.    Oleic  Acid.     Potassium  Oleate.     Wator 

All  soaps  are,  therefore,  salts  formed  by  the  union  of  alkalies  and 
fatty  acids.  The  sodium  soaps  are  generally  hard,  while  the  potas- 
sium soaps  are  soft.  Those  made  with  stearin  and  palmitin  are 
harder  than  those  made  with  olein.  If  the  soap  is  composed  of  lead, 
zinc,  copper,  etc.,  it  is  insoluble  in  water. 

Emulsification.  —  When  a  neutral  oil  is  vigorously  shaken  with 
water  or  other  fluid,  it  is  broken  up  into  minute  globules  that  are 
more  or  less  permanently  suspended  ;  the  permanency  depending  on 
the  nature  of  the  liquid.  The  most  permanent  emulsions  are  those 
made  with  soap  solutions.  The  process  of  emulsification  and  the 
part  played  by  soap,  can  be  readily  observed  by  placing  on  a  few 
cubic  centimeters  of  a  solution  of  sodium  carbonate  0.25  per  cent,  of  a 
small  quantity  of  a  perfectly  neutral  oil  to  which  has  been  added 
2  or  3  per  cent,  of  a  fatty  acid.  The  combination  of  the  acid  and 
the  alkali  at  once  forms  a  soap.  The  energy  set  free  by  this  combina- 
tion rapidly  divides  up  the  oil  into  extremely  minute  globules.  A 
spontaneous  emulsion  is  thus  formed. 

In  addition  to  the  ordinary  fats,  there  are  present  in  different 
tissues  several  compounds  which,  though  usually  regarded  as  fats, 
nevertheless  differ  materially  from  them  in  composition,  containing, 
as  they  do,  both  nitrogen  and  phosphorus.  These  nitrogenized  or 
phosphorized  fats  are  as  follows  : 


Lecithin,  C^HgoN.POj,,  is  found  in  blood  lymph,  red  and  white  cor- 
puscles, nerve  tissue,  yolk  of  eggs,  etc.  When  pure,  it  presents  itself 
generally  under  the  form  of  a  white,  crystalline  powder,  though  some- 
times as  a  white,  waxy  mass.  Lecithin  is  easily  decomposed,  yielding, 
with  various  reagents,  glycero-phosphoric  acid,  cholin  and  stearic  acid. 

Protagon,  QeoHgosNgPOas,  is  found  most  abundantly  in  the  brain 
tissue,  especially  in  the  white  portion.  It  crystallizes  from  warm 
alcoholic  solutions,  on  cooling,  in  the  form  of  white  needles,  generally 
arranged  in  groups.  It  melts  at  200°  C,  and  forms  a  syrupy  liquid. 


CHEMIC   COMPOSITION    OF   THE    HUMAN    BODY.  17 

Cerebrin,  CifHgsN.Os,  is  found  largely  in  the  brain,  in  nerves,  and 
in  pus-corpuscles.  It  is  a  soft,  white,  amorphous  powder,  insoluble 
in  water,  but  swelling  up  like  starch  in  boiling  water.  When  boiled 
with  dilute  acids,  it  is  decomposed,  yielding  a  fermentable  dextro- 
rotary  sugar,  identical  with  galactose.  Cerebrin  may,  therefore,  be 
regarded  as  a  glucosid. 

THE  PROTEIDS. 

The  proteids  constitute  a  group  of  organic  bodies  which  are  found 
in  both  vegetable  and  animal  tissues.  Though  present  in  all  animal 
tissues,  they  are  especially  abundant  in  muscles  and  bones,  where 
they  constitute  twenty  per  cent,  and  thirty  per  cent,  respectively. 
Though  genetically  related,  and  possessing  many  features  in  common, 
the  different  members  of  the  proteid  group  are  distinguished  by 
characteristic  physical  and  chemic  properties. 

The  average  percentage  composition  of  several  proteids  is  shown 
in  the  following  analyses  : 

C.  H.  N.         O.  S. 

Egg-albumin    .  52.9  7.2  15.6     23.9  0.4    (Wiirtz). 

Serum-ablumin  53.0  6.8  16.0     22.29  1.77  (Hammersten). 

Casein     .     .     .  53.3  7.07  15.91   22.03  0.82  (Chittenden  and  Painter). 

Myosin    .     .     .  52.82  7.11  16.77  21.90  1.27  (Chittenden  and  Cummins). 

The  molecular  composition  of  the  proteids  is  not  definitely  known, 
and  the  formulae  which  have  been  suggested  are  therefore  only  ap- 
proximative. Leow  assigns  to  albumin  the  formula  C72H112N18O22S, 
while  Schiitzenberger  raises  the  numbers  to  Ca-ioHsgsNetjO^Sa,  either  of 
which  shows  that  the  proteid  molecule  is  extremely  complex.  As  a 
class,  the  proteids  are  characterized  by  the  following  properties  : 

i.  Indiffusibility. — None  of  the  proteids  normally  assumes  the  crys- 
talline form,  and  hence  they  are  not  capable  of  diffusing  through 
parchment  or  an  animal  membrane.  Peptone,  a  product  of  the 
digestion  of  proteids,  is  an  exception  as  regards  its  diffusibility. 
As  met  with  in  the  body,  all  proteids  are  amorphous,  but  vary  in 
consistence  from  the  liquid  to  the  solid  state.  The  colloid  charac- 
ter of  the  proteids  permits  of  their  separation  and  purification  from 
crystalloid  diffusible  compounds  by  the  process  of  dialysis. 
3 


18  HUMAN    PHYSIOLOGY. 

2.  Solubility. — Some  of  the  proteids  are  soluble  in  water,  others  in 
solutions  of  the  neutral  salts  of  varying  degrees  of  concentration, 
in  strong  acids  and  alkalies.     All  are  insoluble  in  alcohol  and  ether. 

3.  Coagulability. — Under  the  influence  of  heat  and  various  acids  and 
animal  ferments,  the  proteids  readily  pass  from  the  soluble  liquid 
state  to  the  insoluble  solid  state,  attended  by  a  permanent  alteration 
in  their  chemic  composition.     To  this  change  the  term  coagulation 
has  been  given.     The  various  proteids  not  only  coagulate  at  differ- 
ent  temperatures,   but   with    different   chemic   reagents — distinctive 
features  which  permit  not  only  of  their  detection,  but  separation. 
Proteids  are  capable  of  precipitation  without  losing  their  solubility 
by   ammonium   sulphate,   sodium   chlorid   and  magnesium   sulphate. 

4.  Fermentability. — In  the  presence  of  specific  microorganisms — bac- 
teria— the  proteids,  owing  to  their  complexity  and  instability,   are 
prone    to    undergo    disintegration    and    reduction    to    simpler    com- 
pounds.    This   decomposition   or  putrefaction   occurs   most   readily 
when  the  conditions  most  favorable  to  the  growth  of  bacteria  are 
present — viz.,  a  temperature  varying  from  25°  C.  to  40°  C.,  moisture 
and  oxygen.     The  intermediate  as  well  as  the  terminal  products  of 
the  decomposition  of  the  proteids  are  numerous,  and  vary  with  the 
composition   of  the  proteid   and  the   specific  physiologic   action   of 
the    bacteria.      Among    the    intermediate    products    is    a    series    of 
alkaloid   bodies,    some    of   which   possess    marked    toxic   properties, 
known  as  ptomains.     The  toxic  symptoms  which  frequently  follow 
the  ingestion  of  foods  in  various  stages  of  putrefaction  are  to  be 
attributed  to  these  compounds.     The  terminal  products  are  repre- 
sented  by  hydrogen   sulphid,   ammonia,   carbon   dioxid,   fats,   phos- 
phates, nitrates,  etc. 

Color  Tests  for  Proteids. — When  proteids  are  present  in  solution, 
they  may  be  detected  by  the  following  color  reactions — viz. : 

1.  Xanthoproteic.     The  solution  is  boiled  with  nitric  acid  for  several 
minutes,  when  the  proteid  assumes  a  light  yellow  color.     After  the 
solution   has   cooled,   the   addition    of   ammonia   changes   the   color 
to  an  orange  or  amber-red. 

2.  The  rose-red  reaction.     The  solution  is  boiled  with  acid  nitrate  of 
mercury  (Millon's  reagent)  for  a  few  minutes,  when  the  coagulated 
proteid  turns  a  purple-red  color. 

3.  The  blue-violet  reaction.     A  few  drops  of  copper  sulphate  solution 
are  first  added  to  the  proteid  solution,  and  then  an  excess  of  sodium 


CHEMIC   COMPOSITION    OF   THE    HUMAN   BODY.  19 

hydroxid.     A  blue-violet   color  is   produced,   which   deepens   some- 
what on  heating,  but  no  further  change  ensues. 

The  proteids  found  in  the  animal  body,  though  possessing  many 
features  in  common,  are  nevertheless  characterized  by  certain  special 
features  which  not  only  serve  for  their  identification,  but  for  their 
classification  into  well-defined  groups,  as  follows  : 

SIMPLE    PROTEIDS. 

1.  ALBUMINS. 

The  members  of  this  group  are  soluble  in  water,  in  dilute  saline 
solutions,  and  in  saturated  solutions  of  sodium  chlorid  and  magnesium 
sulphate.  They  are  coagulated  by  heat,  and  when  dried  form  an 
amber-colored  mass. 

(a)  Serum-albumin  is  found  in  blood,  lymph,  chyle,  tissue  fluids, 
and  milk.  It  is  obtained  readily  by  precipitation  from  blood- 
serum,  after  the  other  proteids  have  been  removed,  on  the 
addition  of  ammonium  sulphate.  When  freed  from  saline 
constituents,  it  presents  itself  as  a  pale,  amorphous  substance, 
soluble  in  water  and  in  strong  nitric  acid.  It  is  coagulated  at 
a  temperature  of  73°  C.,  as  well  as  by  various  acids — e.  g., 
citric,  picric,  nitric,  etc.  It  has  a  rotary  power  of  — 62.6°. 
(fr)  Egg-albumin. — Though  not  a  constituent  of  the  human  body, 
egg-albumin  resembles  the  foregoing  in  many  respects.  When 
obtained  in  the  solid  form  from  the  white  of  the  egg,  it  is 
a  yellow  mass  without  taste  or  odor.  Though  similar  to  serum- 
albumin,  it  differs  from  it  in  being  precipitated  by  ether,  in 
coagulating  at  54° C,  and  in  having  a  lower  rotary  power, 
—  35-5°. 

2.  GLOBULINS. 

The  members  of  this  group  are  insoluble  in  water  and  in  saturated 
solutions  of  sodium  chlorid  and  magnesium  sulphate  and  ammonium 
sulphate.  They  are  soluble,  however,  in  dilute  saline  solutions — e.  g., 
sodium  chlorid  (one  per  cent.),  potassium  chlorid,  ammonium  chlorid, 
etc.  They  are  coagulated  by  heat. 

(a)  Serum-globulin  or  Paraglobulin. — This  proteid,  as  its  name 
implies,  is  found  in  blood-serum,  though  it  is  present  in  other 
animal  fluids.  When  precipitated  by  magnesium  sulphate  or 
carbon  dioxid,  it  presents  itself  as  a  flocculent  substance,  in- 


20  HUMAN   PHYSIOLOGY. 

soluble  in  water,  soluble  in  dilute  acids  and  alkalies,  and  co- 
agulating at  75°  C. 

(b)  Fibrinogen. — This  proteid  is  found  in  blood  plasma  in  asso- 
ciation   with    serum-globulin    and    serum-albumin.      It    is    also 
present   in   lymph-tissue   fluids   and   in   pathologic   transudates. 
It   can   be   obtained    from   blood-plasma   which   has    been   pre- 
viously treated  with  magnesium  sulphate  on  the  addition  of  a 
saturated  solution   of  sodium  chlorid.     It  is  soluble  in  dilute 
acids  and  alkalies,  and  coagulates  at  56°   C. 

(c)  Myosinogen. — This  proteid  is  a  constituent  of  the  protoplasm 
of  the  muscle-fibers.     During  the  living  condition  it  is  liquid, 
but    after   death    it   readily   undergoes    decomposition    into    an 
insoluble  portion  known  as  myosin  and  a  soluble  albumin.     It 
is  soluble  in  dilute  hydrochloric  acid  and  dilute  alkalies.      It 
coagulates  at  56°  C. 

(d)  Globin. — This  is  a  product  of  the  spontaneous  decomposition 
of  the  coloring  matter  of  the  blood, — hemoglobin, — and  arises 
when  the  latter  is  exposed  to  the  air. 

(e)  Crystallin    or    Globulin. — This    is    obtained    by    passing    a 
stream  of  CO2  through  a  watery  extract  of  the  crystalline  lens. 

3.  DERIVED  ALBUMINS  OR  ALBUMINATES. 

The  proteids  of  this  group  are  derived  from  both  native  albumins 
and  globulins  by  the  gradual  action  of  dilute  acids  and  alkalies,  and 
may  be  regarded  as  compounds  of  a  proteid  with  an  acid  or  an  alkali. 

(a)  Acid-albumin. — This  is  formed  when  a  native  albumin  is 
digested  with  dilute  hydrochloric  acid  (0.2  per  cent.)  or  dilute 
sulphuric  acid  for  some  minutes.  It  is  precipitated  by  neutral- 
ization with  sodium  hydroxid  (o.i  per  cent,  solution).  After 
the  precipitate  is  washed,  it  is  found  to  be  insoluble  in  dis- 
tilled water  and  in  neutral  saline  solutions.  In  acid  solutions 
it  is  not  coagulated  by  heat. 

(&)  Alkali-albumin. — This  is  formed  when  a  native  albumin  is 
treated  with  a  dilute  alkali — e.  g.,  o.i  per  cent,  of  sodium 
hydroxid — for  five  or  ten  minutes.  On  careful  neutralization 
with  dilute  hydrochloric  acid,  it  is  precipitated.  It  is  also 
insoluble  in  distilled  water  and  in  alkaline  solutions ;  it  is  not 
coagulable  by  heat. 


CHEMIC  COMPOSITION   OF  THE   HUMAN   BODY.  21 

4.  COAGULATED   PROTEIDS. 

Although  these  proteids  are  not  found  as  constituents  of  the  animal 
organism,  they  possess  much  interest  on  account  of  their  relation  to 
prepared  foods  and  to  the  digestive  process.  They  are  produced  when 
solutions  of  egg-albumin,  serum-albumin,  or  globulins  are  subjected 
to  a  temperature  of  100°  C.  or  to  the  prolonged  action  of  alcohol. 
They  are  insoluble  in  water,  in  dilute  acids,  and  in  neutral  saline 
solutions.  In  this  same  group  may  be  included  also  those  coagulated 
proteids  which  are  produced  by  the  action  of  animal  ferments  on 
soluble  proteids — e.  g.,  fibrin,  myosin,  casein. 

(a)  Fibrin. — Fibrin  is  derived  from  a  soluble  proteid — fibrinogen 
— by  the  action  of  a  special  ferment.  It  is  not  present  under 
normal  circumstances  in  the  circulating  blood,  but  makes  its 
appearance  after  the  blood  is  withdrawn  from  the  vessels  and 
at  the  time  of  coagulation.  It  can  also  be  obtained  by  whipping 
the  blood  with  a  bundle  of  twigs,  on  which  it  accumulates. 
When  freed  from  blood  by  washing  under  water,  it  is  seen  to 
consist  of  bundles  of  white  elastic  fibers  or  threads.  It  is  in- 
soluble in  water,  in  alcohol,  and  ether.  In  dilute  acids  it  swells, 
becomes  transparent,  and  finally  is  converted  into  acid-albu- 
min. In  dilute  alkalies  a  similar  change  takes  place,  but  the 
resulting  product  is  an  alkali-albumin.  Fibrin  possesses  the 
property  of  decomposing  hydrogen  dioxid,  H2O2 — i.  e,,  liberating 
oxygen,  which  accumulates  in  the  form  of  bubbles  on  the 
fibrin.  On  incineration  fibrin  yields  an  ash  which  contains 
calcium  phosphate  and  magnesium  phosphate. 

(&)  Myosin. — Myosin  develops  in  muscles  after  death  and  is  the 
cause  of  the  stiffening  of  the  muscles.  It  has  been  regarded 
as  a  derivative  of  the  soluble  proteid  myosinogen  alone,  but 
there  is  evidence  that  in  its  formation  both  paramyosinogen 
and  myosinogen  take  part.  It  is  not  definitely  known  whether 
this  is  the  result  of  the  action  of  a  special  ferment  or  not. 
(c)  Casein. — Casein  is  derived  from  the  chief  proteid  of  milk — 
caseinogen — by  the  action  of  a  special  ferment  known  as 
rennin  or  chymosin.  This  ferment  is  a  constituent  of  gas- 
tric juice. 

5.  PROTEOSES  AND  PEPTONES. 

During  the  progress  of  the  digestive  process,  as  it  takes  place  in 
the  stomach  and  intestines,  there  is  produced  by  the  action  of  the 


22  HUMAN    PHYSIOLOGY. 

gastric  and  pancreatic  juices,  out  of  the  proteids  of  the  food,  a  series 
of  new  proteids,,  knows  as  proteoses  and  peptones.  The  chemic  prop- 
erties of  these  substances  will  be  considered  in  connection  with  the 
process  of  digestion. 

CONJUGATED  OR  COMBINED  PROTEIDS. 

The  different  members  of  this  group  are  capable  of  being  de- 
composed by  chemic  methods  into  a  proteid  and  a  non-proteid  sub- 
stance ;  e,  g.,  a  coloring  matter,  a  carbohydrate,  or  a  nuclein.  The 
chemic  character  of  the  non-proteid  substance  furnishes  the  basis 
for  the  following  classification : 

1.  CHROMO-PROTEIDS. 

(a)  Hemoglobin.— Hemoglobin  is  the  coloring  matter  of  the  red 
corpuscles,  of  which  it  constitutes  about  94  per  cent.  It 
possesses  the  power  of  absorbing  oxygen  as  it  passes  through 
the  lung  capillaries  and  of  yielding  it  up  to  the  tissues  as  it 
passes  through  the  tissue  capillaries.  In  the  arterial  blood 
it  is  known  as  oxyhemoglobin,  and  in  the  venous  blood  as 
dioxy-  or  reduced  hemoglobin.  When  hydrolyzed  by  acids 
or  alkalies,  hemoglobin  undergoes  a  cleavage  into  a  proteid, 
globin,  and  a  pigment  hematin. 

(fr)  Myohematin. — Myohematin  is  a  proteid  supposed  to  be 
present  in  muscle.  It  has  never  been  isolated,  hence  its 
chemic  features  are  unknown.  Spectroscopic  examination  in- 
dicates that  it  is  capable  of  absorbing  and  again  yielding 
up  oxygen.  For  this  reason  it  is  believed  to  be  a  derivative 
of  hemoglobin. 

2.  GLUCO-PROTEIDS. 

(a)  Mucin. — Mucin  is  the  proteid  which  gives  the  mucus  secretecl 
by  the  epithelial  cells  of  the  mucous  membranes  and  related 
glands  its  viscid,  tenacious  character.  It  is  also  a  constituent 
of  the  intercellular  substances  of  the  connective  tissues.  It 
is  readily  precipitated  by  acetic  acid.  When  heated  with 
dilute  acids,  mucin  undergoes  a  cleavage  into  a  similar  pro- 
teid and  a  carbohydrate  termed  mucose,  which  is  capable 
of  reducing  Fehling's  solution. 


CHEMIC   COMPOSITION   OF  THE   HUMAN   BODY.  23 

(b*)  Mucoids. — The  mucoids  resemble  the  mucins  though  differ- 
ing from  them  in  solubility  and  in  not  being  precipitable 
from  alkaline  solutions  by  acetic  acid.  They  are  found  in 
the  vitreous  humor,  white  of  egg,  cartilage,  and  in  other 
situations.  They  differ  slightly  one  from  the  other  in  proper- 
ties and  chemic  composition.  They  yield  on  decomposition  a 
carbohydrate. 

3.  NUCLEO-PROTEIDS. 

The  nucleo-proteids  are  obtained  from  the  nuclei  and  cell-substance 
of  tissue-cells.  Chemically  they  are  characterized  by  the  presence  of 
phosphorus  in  relatively  large  amounts.  When  hydrolyzed,  they  sep- 
arate into  a  proteid  and  a  nuclein. 

The  nucleins  derived  from  cell  nuclei  can  be  still  further  separated 
into  a  simpler  proteid  and  nucleic  acid,  which  latter  in  turn  yields 
phosphoric  acid  and  the  so-called  purin  bases,  xanthin,  hypoxanthin, 
aaenin,  and  guanin.  All  nucleins  which  yield  the  purin  bases  are 
termed  true  nucleins. 

The  nucleins  derived  from  caseinogen,  vitellin,  and  probably  cell 
protoplasm  can  be  separated  by  chemic  methods  into  a  proteid  and 
phosphoric  acid  only.  For  the  reason  that  they  do  not  give  origin  to 
purin  bases  they  are  termed  pseudo-  or  paranucleins. 

(a}  Caseinogen. — This  is  the  principal  proteid  of  milk,  in  which 
it  exists  in  association  with  an  alkali,  and  hence  was  formerly 
regarded  as  an  alkali-albumin.  It  is  precipitated  by  acetic 
acid  and  by  magnesium  sulphate.  It  is  coagulated  by  rennet 
— that  is,  separated  into  an  insoluble  proteid,  casein  or  tyrein, 
and  a  soluble  albumin.  Calcium  phosphate  seems  to  be  the 
natural  alkali  necessary  to  this  process,  for  if  it  be  removed 
by  dialysis,  or  precipitated  by  the  addition  of  potassium  oxalate, 
coagulation  does  not  take  place. 

(b)  Vitellin. — Vitellin  is  a  constituent  of  the  vitellis  or  yolk  of 
eggs.  It  differs  from  other  proteids  in  the  fact  that  it  is 
semicrystalline  in  character.  Though  usually  regarded  as  a 
nucleo-proteid  it  is  not  definitely  known  whether  or  not  it 
contains  phosphorus  in  its  composition. 


24  HUMAN   PHYSIOLOGY. 

ALBUMINOIDS. 

The  albuminoids  constitute  a  group  of  substances  similar  to  the 
proteids  in  many  respects,  though  differing  from  them  in  others. 
When  obtained  from  the  tissues,  in  which  they  form  an  organic 
basis,  they  are  found  to  be  amorphous,  colloid,  and  when  decomposed 
yield  products  similar  to  those  of  the  true  proteids.  The  principal 
members  of  the  group  are  as  follows : 

(a)  Collagen,  Ossein. — These  are  two  closely  allied,  if  not  identical, 
substances,  found  respectively  in  the  white  fibrous  connective 
tissue  and  in  bone.  When  the  tendons  of  muscles,  the  liga- 
ments, or  decalcified  bone  are  boiled  for  several  hours,  the 
collagen  and  ossein  are  converted  into  soluble  gelatin,  which, 
when  the  solution  cools,  becomes  solid. 

(fr)  Chondrigen. — This  is  supposed  to  be  the  organic  basis  of 
the  more  permanent  cartilages.  When  they  are  boiled,  they 
yield  a  substance  which  gelatinizes  on  cooling,  and  to  which  the 
name  chondrin  has  been  given. 

(c)  Elastin  is  the  name  given  to  the  substance  composing  the 
fibers  of  the  yellow,  elastic  connective  tissue. 

(d)  Keratin  is  the  substance  found  in  all  horny  and  epidermic 
tissues,  such  as  hairs,  nails,  scales,  etc.     It  differs  from  most 
proteids  in  containing  a  high  percentage  of  sulphur. 

INORGANIC  CONSTITUENTS. 

The  inorganic  compounds  and  mineral  constituents  obtained  from 
the  solids  and  fluids  of  the  body  are  very  numerous,  and,  in  some 
instances,  quite  abundant.  Though  many  of  the  compounds  thus 
obtained  are  undoubtedly  derivatives  of  the  tissues  and  necessary 
to  their  physical  and  physiologic  activity,  others,  in  all  probability, 
are  decomposition  products,  or  transitory  constituents  introduced  with 
the  food.  Of  the  inorganic  compounds,  the  following  are  the  most 
important : 

WATER. 

Water  is  the  most  important  of  the  inorganic  constituents,  as  it  is 
indispensable  to  life.  It  is  present  in  all  the  tissues  and  fluids  with- 
out exception,  varying  from  99  per  cent,  in  the  saliva  to  80  per  cent. 


CHEMIC   COMPOSITION   OF  THE   HUMAN   BODY.  25 

in  the  blood,  75  per  cent,  in  the  muscles  to  2,  per  cent,  in  the  enamel 
of  the  teeth.  The  total  quantity  contained  in  a  body  weighing  75 
kilograms  (165  pounds)  is  52.5  kilograms  (115  pounds).  Much  of 
the  water  exists  in  a  free  condition,  and  forms  the  chief  part  of  the 
fluids,  giving  to  them  their  characteristic  degree  of  fluidity.  Posses- 
sing the  capability  of  holding  in  solution  a  large  number  of  inor- 
ganic as  well  as  some  organic  compounds,  and  being  at  the  same  time 
diffusible,  it  renders  an  interchange  of  materials  between  all  portions 
of  the  body  possible.  It  aids  in  the  absorption  of  new  material  into 
the  blood  and  tissues,  and  at  the  same  time  it  transfers  waste  products 
from  the  tissues  to  the  blood,  from  which  they  are  finally  eliminated, 
along  with  the  water  in  which  they  are  dissolved.  A  portion  of  the 
water  is  chemically  combined  with  other  tissue  constituents,  and 
gives  to  the  tissues  their  characteristic  physical  properties.  The 
consistency,  elasticity,  and  pliability  are,  to  a  large  extent,  conditioned 
by  the  amount  of  water  they  contain.  The  total  quantity  of  water 
eliminated  by  the  kidneys,  lungs,  and  skin  amounts  to  about  three 
kilograms  (6^  pounds). 

CALCIUM  COMPOUNDS. 

Calcium  phosphate,  Ca3(PO4)2,  has  a  very  extensive  distribution 
throughout  the  body.  It  exists  largely  in  the  bones,  teeth,  and  to  a 
slight  extent  in  cartilage,  blood,  and  other  tissues.  Milk  contains 
0.27  per  cent.  The  solidity  of  the  bones  and  teeth  is  almost  entirely 
due  to  the  presence  of  this  salt,  and  is,  therefore,  to  be  regarded  as 
necessary  to  their  structure.  It  enters  into  chemic  union  with  the 
organic  matter,  as  shown  by  the  fact  that  it  can  not  be  separated 
from  it  except  by  chemic  means,  such  as  hydrochloric  acid.  Though 
insoluble  in  water,  it  is  held  in  solution  in  the  blood  and  milk  by  the 
proteid  constituents,  and  in  the  urine  by  the  acid  phosphate  of  soda. 
The  total  quantity  of  calcium  phosphate  which  enters  into  the  for- 
mation of  the  body  has  been  estimated  at  2.5  kilograms.  The  amount 
eliminated  daily  from  the  body  has  been  estimated  at  0.4  gm.,  a  fact 
which  indicates  that  nutritive  changes  do  not  take  place  with  much 
rapidity  in  those  tissues  in  which  it  is  contained. 

Calcium  carbonate,  CaCO3,  is  present  in  practically  the  same  situ- 
ations in  the  body  as  the  phosphate,  and  plays  essentially  the  same 
role.  It  is,  however,  found  in  the  crystalline  form,  aggregated  in 
small  masses  in  the  internal  ear,  forming  the  otoliths,  or  ear  stones. 


HUMAN    PHYSIOLOGY. 

Though  insoluble,  it  is  held  in  solution  by  the  carbonic  acid  diffused 
through  the  fluids. 

Calcium  chlorid,  CaF2,  is  found  in  bones  and  teeth. 

SODIUM  COMPOUNDS. 

Sodium  chloiid,  NaCl,  is  present  in  all  the  tissues  and  fluids  of 
the  body,  but  especially  in  the  blood,  0.6  per  cent.  ;  lymph,  0.5,  and 
pancreatic  juice,  0.25  per  cent.  The  entire  quantity  in  the  body  has 
been  estimated  at  about  200  gm.  Sodium  chlorid  is  of  much  impor- 
tance in  the  body,  as  it  determines  and  regulates  to  a  large  extent 
the  phenomena  of  diffusion  which  are  there  constantly  taking  place. 
This  is  illustrated  by  the  fact  that  a  solution  of  albumin  placed  in 
the  rectum  without  the  addition  of  this  salt  will  not  be  absorbed. 
When  the  salt  is  added,  absorption  takes  place.  The  ingested  water 
is  absorbed  into  the  blood  largely  in  consequence  of  the  percentage 
of  this  salt  which  it  contains.  The  normal  percentage  of  sodium 
chlorid  in  the  blood-plasma  assists  in  maintaining  the  shape  and 
structure  of  the  red  blood-corpuscles  by  determining  the  amount  of 
water  entering  into  their  composition.  The  same  is  true  of  other 
tissue  elements. 

Sodium  chlorid  also  influences  the  general  nutritive  process,  in- 
creasing the  disintegration  of  the  proteids,  as  shown  by  the  increased 
amount  of  urea  excreted.  During  its  existence  in  the  body  it  under- 
goes some  chemic  transformations  or  decompositions,  yielding  its 
chlorid  to  form  potassium  chlorid  of  the  blood-corpuscles  and  muscles 
and  to  form  the  hydrochloric  acid  of  the  gastric  juice. 

Sodium  phosphate,  Na2HPO4,  is  found  in  all  solids  and  fluids  of 
the  body,  to  which,  with  but  few  exceptions,  it  imparts  an  alkaline 
reaction.  This  is  especially  true  of  blood,  lymph,  and  tissue  fluids 
generally.  It  is  essential  to  physiologic  action  that  all  tissue  elements 
should  be  bathed  by  an  alkaline  medium. 

Sodium  carbonate,  Na2CO3,  is  generally  found  in.  association  with 
the  preceding  salt.  As  it  is  also  an  alkaline  compound,  it  assists 
in  giving  to  the  blood  and  lymph  their  characteristic  alkalinity.  In 
carnivorous  animals  the  sodium  phosphate  is  the  more  abundant, 
while  in  the  herbivorous  animals  the  reverse  is  true. 

Sodium  sulphate,  Na2SO4,  is  present  in  many  of  the  tissues  and 
fluids,  especially  the  urine.  Though  introduced  in  the  food,  it  is  also, 


CHEMIC   COMPOSITION    OF   THE   HUMAN    BODY.  27 

in   all  probability,   formed   in   the  body   from   the   decomposition   and 
oxidation  of  the  proteids. 

POTASSIUM  COMPOUNDS. 

Potassium  chlorid,  KC1,  is  met  with  in  association  with  sodium 
chlorid  in  almost  all  situations  in  the  body.  It  preponderates,  how- 
ever, in  the  tissue  elements,  especially  in  the  muscle  tissue,  nerve 
tissue,  and  red  corpuscles.  The  plasma-  with  which  these  structures 
are  bathed  contains  but  a  very  small  amount  of  this  salt,  but,  as 
previously  stated,  a  relatively  large  quantity  of  sodium  chlorid. 
Though  introduced  to  some  extent  in  the  food,  it  is  very  likely  that 
it  is  also  formed  through  the  decomposition  of  the  sodium  chlorid. 

Potassium  phosphate,  K2HPO4,  is  found  in  association  with  sodium 
phosphate  in  all  the  fluids  and  solids.  As  it  has  similar  chemic 
properties,  its  functions  are  practically  the  same. 

Potassium  carbonate,  K2CO3,  is  generally  found  with  the  preced- 
ing salt. 

MAGNESIUM  COMPOUNDS. 

Magnesium  phosphate,  Mg3(PO4)2,  is  found  in  all  tissues,  in  asso- 
ciation with  calcium  phosphate,  though  in  much  smaller  quantity. 

Magnesium  carbonate,  MgCO3,  occurs  only  in  traces  in  the  blood. 
Both   of  these   compounds   have    functions   similar   to   the   calcium 
compounds,  and  exist,  in  all  probability,  under  similar  conditions. 

IRON  COMPOUNDS. 

Iron  is  a  constituent  of  the  coloring  matter  of  the  blood.  Traces, 
however,  are  also  found  in  lymph,  bile,  gastric  juice,  and  in  the 
pigment  of  the  eyes,  skin,  and  hair.  The  amount  of  iron  contained 
in  a  body  weighing  seventy-five  kilogram^  is  about  three  gm.  It 
exists  under  various  forms — e.  g,,  ferric  oxid,  ferrous  oxid,  and  in 
combination  with  organic  compounds. 

Chemic  analysis  thus  shows  that  the  chemic  elements  into  which 
the  compounds  may  be  resolved  by  an  ultimate  analysis  do  not  exist 
in  the  body  in  a  free  state,  but  only  in  combination,  and  in  charac- 
teristic proportions,  to  form  compounds  whose  properties  are  the 
resultant  of  those  of  the  elements.  Of  the  four  principal  elements 
which  make  up  ninety-seven  per  cent,  of  the  body,  O,  H,  N  are 
extremely  mobile,  elastic,  and  possessed  of  great  atomic  heat.  C,  H, 


28  HUMAN   PHYSIOLOGY. 

N  are  distinguished  for  the  narrow  range  of  their  affinities,  and  for 
their  chemic  inertia.  C  possesses  the  great  atomic  cohesion.  O  is 
noted  for  the  number  and  intensity  of  its  combinations. 

As  the  properties  of  the  compounds  formed  by  the  union  of  ele- 
ments must  be  the  resultants  of  the  properties  of  the  elements  them- 
selves, it  follows  that  the  ternary  compounds,  starches,  sugars,  and 
fats  must  possess  more  or  less  inertia,  and  at  the  same  time  insta- 
bility ;  while  in  the  more  complex  proteids,  in  which  sulphur  and 
phosphorus  are  frequently  combined  with  the  four  principal  elements, 
molecular  instability  attains  its  maximum.  As  all  the  foregoing  com- 
pounds possess  in  varying  degrees  the  properties  of  inertia  and 
instability,  it  follows  that  living  matter  must  possess  corresponding 
properties,  and  the  capability  of  undergoing  unceasingly  a  series  of 
chemic  changes,  both  of  composition  and  decomposition,  in  response 
to  the  chemic  and  physical  influences  by  which  it  is  surrounded, 
and  which  underlie  all  the  phenomena  of  life. 

PRINCIPLES  OF  DISSIMILATION. 

In  addition  to  the  previously  mentioned  compounds, — viz.,  carbo- 
hydrates,   fats,   proteids,   and   inorganic   salts, — there   is   obtained   by 
chemic  analysis  from  the  tissues  and  fluids  of  the  body : 
i.  A  number  of  organic  acids,  such  as  acetic,  lactic,  oxalic,  butyric, 

propionic,  etc.,  in  combination  with  alkaline  and  earthy  bases. 
2..  Organic  compounds,  such  as  alcohol,  glycerin,  cholesterin. 

3.  Pigments,  such  as  those  found  in  bile  and  urine. 

4.  Crystallizable  nitrogenized  bodies,  such  as  urea,  uric  acid,  xanthin, 
hippuric  acid,  creatin,  creatinin,  etc. 

While  some  few  of  these  compounds  may  possibly  be  regarded  as 
necessary  to  the  physiologic  integrity  of  the  tissues  and  fluids,  the 
majority  of  them  are  to  4>e  regarded  as  products  of  dissimilation  of 
the  "tissues  and  foods  in  consequence  of  functional  activity,  and 
represent  stages  in  their  reduction  to  simpler  forms  previous  to  being 
eliminated  from  the  body. 


PHYSIOLOGY   OF   THE   CELL.  29 


PHYSIOLOGY    OF    THE    CELL. 

A  microscopic  analysis  of  the  tissues  shows  that  they  can  be 
resolved  into  simpler  elements,  termed  cells,  which  may,  therefore,  be 
regarded  as  the  primary  units  of  structure.  Though  cells  vary  con- 
siderably in  shape,  size,  and  chemic  composition  in  the  different 
tissues  of  the  adult  body,  they  are,  nevertheless,  descendants  from 
typical  cells,  known  as  embryonic  or  undifferentiated  cells,  examples 
of  which  are  the  leukocytes  of  the  blood  and  lymph  and  the  first 
offspring  of  the  fertilized  ovum.  Ascending  the  line  of  embryonic 
development,  it  will  be  found  that  every  organized  body  originates 
in  a  single  cell — the  ovum.  As  the  cell  is  the  elementary  unit  of  all 
tissues,  the  function  of  each  tissue  must  be  referred  to  the  function 
of  the  cell.  Hence  the  cell  may  be  defined  as  the  primary  anatomic 
and  physiologic  unit  of  the  organic  world,  to  which  every  exhibition 
of  life,  whether  normal  or  abnormal,  is  to  be  referred. 

Structure  of  Cells. — Though  cells  vary  in  shape  and  size  and  in- 
trenal  structure  in  different  portions  of  the  body,  a  typical  cell  may 
be  said  to  consist  mainly  of  a  gelatinous  substance  forming  the  body 
of  the  cell,  termed  protoplasm  or  bioplasm,  in  which  is  embedded  a 
smaller  spheric  body,  the  nucleus.  The  shape  of  the  adult  cell 
varies  according  to  the  tissue  in  which  it  is  found ;  when  young  and 
free  to  move  in  a  fluid  medium,  the  cell  assumes  a  spheric  form, 
•but  when  subjected  to  pressure,  may  become  cylindric,  fusiform, 
polygonal,  or  stellate.  Cells  vary  in  size  within  wide  limits,  ranging 
from^J^  of  an  inch,  the  diameter  of  a  red  blood-corpuscle,  to  ^J7  of 
an  inch,  the  diameter  of  the  large  cells  in  the  gray  matter  of  the 
spinal  cord.  (See  Fig.  2.) 

The  cell  protoplasm  consists  of  a  soft,  semifluid,  gelatinous  material, 
varying  somewhat  in  appearance  in  different  tissues.  Though  fre- 
quently homogeneous,  it  often  exhibits  a  finely  granular  appearance 
under  medium  powers  of  the  microscope.  Young  cells  consist  almost 
entirely  of  clear  protoplasm,  mature  cells  contain,  according  to  the 
tissue  in  which  they  are  found,  material  of  an  entirely  different  char- 
acter— e.  g.,  small  globules  of  fat,  granules  of  glycogen,  mucigen, 
pigments,  digestive  ferments,  etc.  Under  high  powers  of  the  micro- 
scope the  cell  protoplasm  is  found  to  be  pervaded  by  a  network  of 
fibers,  termed  spongioplasm,  in  the  meshes  of  which  is  contained  a 


30 


HUMAN    PHYSIOLOGY. 


clearer  and  more  fluent  substance,  the  hyaloplasm.  The  relative 
amount  of  these  two  constituents  varies  in  different  cells,  the  propor- 
tion of  hyaloplasm  being  usually  greater  in  young  cells.  The  arrange- 
ment of  the  fibers  forming  the  spongioplasm  also  varies,  the  fibers 
having  sometimes  a  radial  direction,  in  others  a  concentric  disposition, 
but  most  frequently  being  distributed  evenly  in  all  directions.  In 
many  cells  the  outer  portion  of  the  cell  protoplasm  undergoes  chemic 


Nuclear    mem- 
brane. 


Linin. 


Nuclear    fluid 
(matrix). 


Nucleolus. 


Chromatin    "*' 
cords    (nuclear 
network) . 

Nodal  enlarge-   '*' 
ments  of  the 
chromatin. 


Cell  membrane. 


Spongioplasm. 
Hyaloplasm. 

Foreign    inclo- 
sures. 


FIG.   2. — DIAGRAM   OF  A   CELL. 
Microsomes  and  spongioplasm  are  only  partly  drawn. 

changes  and  is  transformed  into  a  thin,  transparent,  homogeneous 
membrane, — the  cell  membrane, — which  completely  incloses  the  cell 
substance.  The  cell  membrane  is  permeable  to  water  and  watery 
solutions  of  various  inorganic  and  organic  substances.  It  is,  how- 
ever, not  an  essential  part  of  the  cell. 

The  nucleus  is  a  small  vesicular  body  embedded  in  the  protoplasm 
near  the  center  of  the  cell.  In  the  resting  condition  of  the  cell  it 
consists  of  a  distinct  membrane,  composed  of  amphipyrenin,  inclosing 
the  nuclear  contents.  The  latter  consists  of  a  homogeneous  amor- 
phous substance, — the  nuclear  matrix, — in  which  is  embedded  the 
nuclear  network.  It  can  often  be  seen  that  a  portion  of  one  side  of 


PHYSIOLOGY    OF   THE   CELL.  31 

the  nucleus,  called  the  pole,  is  free  from  this  network.  The  main 
cords  of  the  network  are  arranged  as  V-shaped  loops  about  it.  These 
main  cords  send  out  secondary  branches  or  twigs,  which,  uniting  with 
one  another,  complete  the  network.  The  nuclear  cords  are  composed 
of  granules  of  chromatin, — so  called  because  of  its  affinity  for  certain 
staining  materials, — held  together  by  an  achromatin  substance  known 
as  linin.  Besides  the  nuclear  network,  there  are  embedded  in  the 
nuclear  matrix  one  or  more  small  bodies  composed  of  pyrenin,  known 
as  nucleoli.  At  the  pole  of  the  nucleus,  either  within  or  just  without 
in  the  protoplasm,  is  a  small  body,  the  centrosome,  or  pole  corpuscle. 

Chemic  Composition  of  the  Cell. — The  composition  of  living  pro- 
toplasm is  difficult  of  determination,  for  the  reason  that  all  chemic 
and  physical  methods  employed  for  its  analysis  destroy  its  vitality, 
and  the  products  obtained  are  peculiar  to  dead  rather  than  to  living 
matter.  Moreover,  as  protoplasm  is  the  seat  of  constructive  and 
destructive  processes,  it  is  not  easy  to  determine  whether  the  products 
of  analysis  are  crude  food  constituents  or  cleavage  or  disintegration 
products.  Nevertheless,  chemic  investigations  have  shown  that  even 
in  the  living  condition  protoplasm  is  a  highly  complex  compound — the 
resultant  of  the  intimate  union  of  many  different  substances.  About 
seventy-five  per  cent,  of  protoplasm  consists  of  water  and  twenty-five 
per  cent,  of  solids,  of  which  the  more  important  compounds  are 
various  nucleo-proteids  (characterized  by  their  large  percentage  of 
phosphorus),  globulins,  traces  of  lecithin,  cholesterin,  and  frequently 
fat  and  carbohydrates.  Inorganic  salts,  especially  the  potassium, 
sodium,  and  calcium  chlorids  and  phosphates,  are  almost  invariable 
and  essential  constituents. 

MANIFESTATIONS  OF  CELL  LIFE. 

Growth,  Nutrition. — All  cells  exhibit  the  three  fundamental  prop- 
erties of  life — viz.,  growth,  nutrition,  reproduction.  All  cells  when 
newly  reproduced  are  extremely  small,  but  by  the  absorption  of  nutri- 
tive material  from  their  surrounding  medium,  they  gradually  grow 
until  they  attain  their  mature  size.  This  is  accomplished  by  the 
power  which  living  material  possesses  of  transforming,  vitalizing, 
and  organizing  crude  nutritive  material,  through  a  series  of  upward 
changes,  into  material  similar  to  itself.  To  all  these  changes  the 
term  assimilation,  or  anabolism,  has  been  given.  Some  of  the  ab- 
sorbed material,  in  all  probability,  never  becomes  an  integral  part 


32  HUMAN   PHYSIOLOGY. 

of  the  living  bioplasm,  but  undergoes  disruption  and  oxidation,  giving 
rise  at  once  to  heat  and  force.  Coincident  with  the  assimilative 
processes,  a  series  of  disintegrative  processes  is  constantly  taking 
place,  whereby  the  living  material  is  reduced,  through  a  series  of 
downward  chemic  changes,  to  simpler  compounds,  such  as  water, 
carbon  dioxid,  urea,  etc.  To  all  these  downward  changes  the  term 
dissimilation,  or  katabolism,  has  been  given.  As  a  result,  also,  of 
these  various  changes,  the  protoplasm  gives  rise  to  the  production 
of  material  of  an  entirely  different  character,  such  as  globules 
of  fat,  granules  of  glycogen,  mucigen,  digestive  ferments,  etc.  The 
sum  total  of  all  changes  which  go  on  in  the  cell,  both  assimilative 
and  dissimilative,  are  embraced  under  the  general  term  nutrition,  or 
metabolism.  Every  cell  presents  in  its  nutritive  activities  an  epitome 
of  the  nutritive  activities  of  the  body  as  a  whole. 

Physiologic  Properties  of  Protoplasm. — All  living  protoplasm  pos- 
sesses properties  which  serve  to  distinguish  and  characterize  it — viz., 
irritability,  conductivity,  and  motility. 

Irritability,  or  the  power  of  reacting  in  a  definite  manner  to  some 
form  of  external  excitation,  whether  mechanical,  chemic,  or  electric, 
is  a  fundamental  property  of  all  living  protoplasm.  The  character  and 
extent  of  the  reaction  will  vary,  and  will  depend  both  on  the  nature 
of  the  protoplasm  and  the  character  and  strength  of  the  stimulus.  If 
the  protoplasm  be  muscle,  the  response  will  be  a  contraction  ;  if  it 
be  gland,  the  response  will  be  secretion ;  if  it  be  nerve,  the  response 
will  be  a  sensation  or  some  other  form  of  nerve  activity. 

Conductivity,  or  the  power  of  transmitting  molecular  disturbances 
arising  at  one  point  to  all  portions  of  the  irritable  material,  is  also  a 
characteristic  feature  of  all  protoplasm.  This  power,  however  is 
best  developed  in  that  form  of  protoplasm  found  in  nerves,  which 
serves  to  transmit,  with  extreme  rapidity,  molecular  disturbances 
arising  at  the  periphery  to  the  brain,  as  well  as  in  the  reverse  direc- 
tion. Muscle  protoplasm  also  possesses  the  same  power  in  a  high 
degree. 

Motility,  or  the  power  of  executing  apparently  spontaneous  move- 
ments, is  exhibited  by  many  forms  of  cell  protoplasm.  In  addition 
to  the  molecular  movements  which  take  place  in  certain  cells,  other 
forms  of  movement  are  exhibited,  more  or  less  constantly,  by  many  cells 
in  the  animal  body — e.  g.,  the  waving  of  cilia,  the  ameboid  movements 
and  migrations  of  white  blood  corpuscles,  the  activities  of  sperma- 


PHYSIOLOGY   OF   THE   CELL. 


tozooids,  the  projections  of  pseudopodia,  etc.  These  movements, 
arising  without  any  recognizable  cause,  are  frequently  spoken  of  as 
spontaneous.  Strictly  speaking,  however,  all  protoplasmic  move- 
ment is  the  resultant  of  natural  causes,  the  true  nature  of  which  is 
beyond  the  reach  of  present  methods  of  investigation. 

Reproduction. — Cells  reproduce  themselves  in  the  higher  animals 
in  two  ways — by  direct  division  and  by  indirect  division,  or  karyo- 
kinesis.  In  the  former  the  nucleus  becomes  constricted,  and  divides 
without  any  special  grouping  of  the  nuclear  elements.  It  is  prob- 


Close  Skein     Loose  Skein  (viewed 
(viewed  from  from  above — i.  e.,  from 


the  side). 
Polar  field. 


the  pole). 


Mother  Stars  (viewed  from  the  side). 


Mother  Star  (viewed  Daughter  Star, 
from  above). 


Beginning.          Completed. 
Division  of  the  Protoplasm. 


FIG.  3. — KARYOKINETIC  FIGURES  OBSERVED  IN  THE  EPITHELIUM  OF  THE  ORAL 
CAVITY  OF  A  SALAMANDER. 

The  picture  in  the  upper  right-hand  corner  is  from  a  section  through  a  divid- 
ing egg  of  Siredon  pisciformis.  Neither  the  centrosomes  nor  the  first 
stages  of  the  development  of  the  spindle  can  be  seen  by  this  magnification. 
X  560. 


able  that  this  occurs  only  in  disintegrating  cells,  and  never  in  a 
physiologic  multiplication.  In  division  by  karyokinesis  (Fig.  3)  there 
is  a  progressive  rearranging  and  definite  grouping  of  the  nucleus, 
the  result  of  which  changes  is  the  division  of  the  centrosome,  the 


34  HUMAN   PHYSIOLOGY. 

chromatin,  and  the  rest  of  the  nucleus  into  two  equal  portions,  which 
form  the  nuclei.  Following  the  division  of  the  nuclei,  the  protoplasm 
divides.  The  process  may  be  divided  into  three  phases  : 

1.  Prophase. — The   centrosome,   at  first   small   and   lying   within   the 
nucleus,  increases  in  size  and  moves  into  the  protoplasm,  where  it 
lies   near   the    nucleus,    surrounded   by    a   clear   zone,    from    which 
delicate  threads   radiate  through   an  area  known  as  the  attraction 
sphere.     The   nucleus   enlarges   and  becomes   richer   in   chromatin. 
The  lateral  twigs  of  the  chromatin  cords  are  drawn  in,  while  the 
main  cords  become  much  contorted.     These  cords  have  a  general 
direction  transverse  to   the  long  axis   of  the  cell,   and  parallel   to 
the   plane    of   future    cleavage.      They    are    seen   as    V-shaped   seg- 
ments   or   loops,    chromosomes,   having   their   closed    ends    directed 
toward    a   common    center,    the   polar   Held,   while    the    other    ends 
interdigitate    on    the    opposite    side    of   the    nucleus — the    anti-pole. 
The  .polar   field   corresponds   to   the   area   occupied   by   the   centro- 
some.    This  arrangement  is  known  as  the  close  skein;  but  as  the 

-  process  goes  on,  the  chromosomes  become  thicker,  shorter  and  less 
contorted,  producing  a  much  looser  arrangement,  known  as  the 
loose  skein.  During  the  formation  of  the  loose  skein,  the  centro- 
some divides  into  two  portions,  which  move  apart  to  positions  at 
the  opposite  ends  of  the  long  axis  of  the  nucleus.  At  the  same 
time  delicate  achromatin  fibers  make  their  appearance,  arranged 
in  the  form  of  a  double  cone,  the  apices  of  which  correspond  in 
position  to  the  centrosome.  This  is  known  as  the  nuclear  spindle. 
During  the  prophase  the  nuclear  membrane  and  the  nucleoli  dis- 
appear. 

2.  The   Metaphase. — The   two   centrosomes   are   at   opposite   ends   of 
the   long   axis   of   the   nucleus,    each    surrounded   by   an   attraction 
sphere,  now  called  the  polar  radiation.     The  chromosomes  become 
yet  shorter  and  thicker,  and  move  toward  the  equator  of  the  nucleus, 
where  they  lie  with  their  closed  ends  toward  the  axis,  presenting 
the  appearance,  when  seen  from  the  poles,  of  a  star, — the  so-called 
mother  star,  or  monaster.     While   moving  toward  the   equator   of 
the  nucleus,  and  often  earlier,  each  chromosome  undergoes  longi- 
tudinal  cleavage,   the   sister  loops   remaining  together   for   a   time. 
Upon  the  completion  of  the  monaster,  one  loop  of  each  pair  passes 
to   each   pole   of  the   nucleus,   guided,   and   perhaps    drawn   by  the 
threads  of  the  nuclear  spindle.     The  separation  of  the  sister  seg- 
ments  begins    at   their   apices,    and   as    the   open    ends    are    drawn 


HISTOLOGY   OF   EPITHELIAL   AND   CONNECTIVE  TISSUES.  35 

apart  they  remain  connected  by  delicate  achromatin  filaments 
drawn  out  from  the  chromosomes.  This  separation  of  the  daughter 
chromosomes,  and  their  movement  toward  the  daughter  centro- 
somes,  is  called  metakinesis.  As  they  approach  their  destination, 
we  have  the  appearance  of  two  stars  in  the  nucleus — the  daughter 
stars,  or  diasters. 

.  Anaphase. — The  daughter  stars  undergo,  in  reverse  order,  much 
the  same  changes  that  the  mother  star  passed  through.  The 
chromosomes  become  much  convoluted,  and  perhaps  united  to  one 
another,  the  lateral  twigs  appear,  and  the  chromatin  resumes  the 
appearance  of  the  resting  nucleus.  The  nuclear  spindle,  with 
most  of  the  polar  radiation,  disappears,  and  the  nucleoli  and  the 
nuclear  membrane  reappear,  thus  forming  two  complete  daughter 
nuclei.  Meanwhile  the  protoplasm  becomes  constricted  midway 
between  the  young  nuclei.  This  constriction  gradually  deepens 
until  the  original  cell  is  divided,  with  the  formation  of  two  com- 
plete cells. 


HISTOLOGY     OF     THE     EPITHELIAL     AND 
CONNECTIVE     TISSUES. 

i.     EPITHELIAL    TISSUE. 

The  epithelial  tissue  consists  of  one  or  more  layers  of  cells  resting 
on  a  homogeneous  membrane,  the  other  side  of  which  is  abundantly 
supplied  with  blood-vessels  and  nerves.  The  form  of  the  epithelial 
cell  varies  in  different  situations,  and  may  be  flattened,  cuboid, 
spheroid,  or  columnar.  The  form  of  the  cell  in  all  instances  is  re- 
lated to  some  specific  function.  When  arranged  in  layers  or  strata, 
the  cells  are  cemented  together  by  an  intercellular  substance — mucin. 

The  epithelial  tissue  forms  a  continuous  covering  for  the  surfaces 
of  the  body.  The  external  investment  (the  skin)  and  the  internal 
investment  (the  mucous  membrane,  which  lines  the  entire  alimentary 
canal  and  its  associated  body  cavities)  are  both  formed,  in  all  situa- 
tions, by  the  homogeneous  basement  membrane,  covered  with  one  or 
more  layers  of  cells.  All  materials,  therefore,  whether  nutritive, 
secretory,  or  excretory,  must  pass  through  epithelial  cells  before 
they  can  enter  into  the  formation  of  the  blood  or  be  eliminated  from 
it.  The  nutrition  of  the  epithelial  tissue  is  maintained  by  the 


36  HUMAN    PHYSIOLOGY. 

nutritive  material  derived  from  the  blood  diffusing  itself  into  and 
through  the  basement  membrane.  Chemically,  the  epithelial  cells  of 
the  epidermis — hair,  nails,  etc. — are  composed  of  an  albuminoid 
material  (keratin),  a  small  quantity  of  water,  and  inorganic  salts. 
In  other  situations,  e'specially  on  the  mucous  membranes,  the  cells 
consist  largely  of  mucin,  in  association  with  other  proteids.  The 
consistency  of  epithelium  varies  in  accordance  with  external  influ- 
ences, such  as  the  presence  or  absence  of  moisture,  pressure,  friction, 
etc.  This  is  well  seen  in  the  skin  of  the  palms  of  the  hands  and  the 
soles  of  the  feet — situations  where  it  acquires  its  greatest  density. 
In  the  alimentary  canal,  in  the  lungs,  and  in  other  cavities,  where 
the  reverse  conditions  prevail,  the  epithelium  is  extremely  soft. 
Epithelial  tissues  also  possess  varying  degrees  of  cohesion  and  elas- 
ticity— physical  properties  which  enable  them  to  resist  considerable 
pressure  and  distension  without  having  their  physiologic  integrity 
destroyed.  Inasmuch  as  these  tissues  are  poor  conductors  of  heat, 
they  assist  in  preventing  too  rapid  radiation  of  heat  from  the  body, 
and  cooperate  with  other  mechanisms  in  maintaining  the  normal 
temperature.  The  physiologic  activity  of  all  epithelial  tissue  depends 
on  a  due  supply  of  nutritive  material  derived  from  the  blood,  which 
not  only  maintains  its  own  nutrition,  but  affords  those  materials 
out  of  which  are  formed  the  secretions  of  the  glands,  whether  of 
the  skin  or  mucous  membrane. 

Functions  of  Epithelial  Tissue. — In  succeeding  chapters  the  form, 
chemic    composition,    and   functions    of   epithelial   cells   will   be    con- 
sidered in  connection  with  the  functions  of  the  organs  of  which  they 
constitute  a  part.     In  this  connection  it  may  be  stated  in  a  general 
way  that  the  functions  of  the  epithelial  tissues  are : 
i.  To  serve  on  the  surface  of  the  body  as  a  protective  covering  to  the 
underlying  structures   which  collectively   form  -  the  true   skin,  thus 
protecting  them  from  the  injurious  influences  of  moisture,  air,  dust, 
microorganisms,  etc.,  which  would  otherwise  impair  their  vitality. 
Wherever   continuous   pressure   is   applied   to   the   skin,   as   on   the 
palms  of  the  hands  and  soles  of  the  feet,  the  epithelium  increases 
in  thickness  and  density,  and  thus  prevents  undue  pressure  on  the 
nerves  of  the  true  skin.     The  density  of  the  epidermis  enables  it  to 
resist,  within  limits,  the  injurious  influences  of  acids,  alkalies,  and 
poisons. 


HISTOLOGY   OP  EPITHELIAL  AND  CONNECTIVE  TISSUES.  37 

2.  To  promote  absorption.     Inasmuch  as  the  skin  and  mucous  mem- 
branes cover  the  surfaces  of  the  body,  it  is  obvious  that  all  nutritive 
material  entering  the  body  must  first  traverse  the  epithelial  tissue. 
Owing  to  their  density,  however,  the  epithelial  cells  covering  the 
skin  play  but   a   feeble   role  as   absorbing  agents   in   man   and  the 
higher  animals.     The  epithelium  of  the  mucous  membrane  of  the 
alimentary  canal,  particularly  that  of  the  small  intestine,  is  especi- 
ally   adapted,    from    its    situation,    consistency,    and    properties,    to 
play  the   chief  role   in   the   absorption   of  new   materials   into   the 
blood.     The  epithelium  lining  the  air-vesicles  of  the  lungs   is   en- 
gaged in  promoting  the  absorption  of  oxygen   and  the  exhalation 
of  carbon  dioxid. 

3.  To    form    secretions   and   excretions.      Each   secretory   gland    con- 
nected with  the  surfaces   of  the  body  is  lined  by  epithelial   cells, 
which   are   actively    concerned    in    the    formation   of   the    secretion 
peculiar  to  the  gland.     Each  excretory  organ  is  similarly  provided 
with  epithelial  cells,  which  are  engaged  either  in  the  production  of 
the  constituents  of  the  excretion  or  in  their  removal  from  the  blood. 

2.    THE   CONNECTIVE   TISSUES. 

The  connective  tissues,  in  their  collective  capacity,  constitute  a 
framework  which  pervades  the  body  in  all  directions,  and,  as  the 
name  implies,  serve  as  a  bond  of  connection  between  the  individual 
parts,  at  the  same  time  affording  a  basis  of  support  for  the  muscle, 
nerve,  and  gland  tissues.  The  connective-tissue  group  includes  a 
number  of  varieties,  among  which  may  be  mentioned  the  areolar, 
adipose,  retiform,  white  fibrous,  yellow  elastic,  cartilaginous  and  os- 
seous. Notwithstanding  their  apparent  diversity,  they  possess  many 
points  of  similarity.  They  have  a  common  origin,  developing  from 
the  same  embryonic  material ;  they  have  much  the  same  structure, 
passing  imperceptibly  into  one  another,  and  perform  practically  the 
same  functions. 

Areolar  Tissue. — This  variety  is  found  widely  distributed  through- 
out the  body.  It  serves  to  unite  the  skin  and  mucous  membrane  to 
the  structures  on  which  they  rest ;  to  form  sheaths  for  the  support 
of  blood-vessels,  nerves,  and  lymphatics  ;  to  unite  into  compact  masses 
the  muscular  tissue  of  the  body,  etc.  Examined  with  the  naked 
eye,  it  presents  the  appearance  of  being  composed  of  bundles  of  fine 
fibers  interlacing  in  every  direction.  In  the  embryonic  state  the  ele- 


HUMAN   PHYSIOLOGY. 

ments  of  this  form  of  connective  tissue  are  united  by  a  ground 
substance,  gelatinous  in  character.  In  the  adult  state  this  sub- 
stance shrinks  and  largely  disappears,  leaving  intercommunicating 
spaces  of  varying  size  and  shape,  from  which  the  tissue  takes  its 
name.  -When  subjected  to  the  action  of  various  reagents,  and 
examined  microscopically,  the  bundles  can  be  shown  to  consist  of 
extremely  delicate,  colorless,  transparent,  wavy  fibers,  which  are 
cemented  together  by  a  ground  substance  composed  largely  of 
mucin.  Other  fibers  are  also  observed,  which  are  distinguished  by 
a  straight  course,  a  sharp,  well-defined  outline,  a  tendency  to  branch 
and  unite  with  adjoining  fibers,  and  to  curl  up  at  their  extremities 
when  torn.  From  their  color  and  elasticity  they  are  known  as  yellow 
elastic  fibers.  Distributed  throughout  the  meshes  of  the  areolar  tissue 
are  found  flattened,  irregularly  branched,  or  stellate  corpuscles,  con- 
nective-tissue corpuscles,  plasma  cells,  and  granule  cells. 

Adipose  Tissue. — This  tissue,  which  exists  very  generally  through- 
out the  body,  though  found  most  abundantly  beneath  the  skin,  around 
the  kidneys,  arnd  in  the  bones,  is  practically  but  a  modification  of 
areolar  tissue.  In  these  situations  it  presents  itself  in  small  masses  or 
lobules  of  varying  size  and  shape,  surrounded  and  penetrated  by  the 
fibers  of  connective  tissue.  Microscopic  examination  shows  that 
these  masses  consist  of  small  vesicles  or  cells,  round,  oval,  or  poly- 
hedral in  shape,  depending  somewhat  on  pressure.  Each  vesicle  con- 
sists of  a  thin,  colorless,  protoplasmic  membrane,  thickened  at  one 
point,  in  which  a  nucleus  can  usually  be  detected.  This  membrane 
incloses  a  globule  of  fat,  which  during  life  is  in  the  liquid  state. 
It  is  composed  of  olein,  stearin,  and  palmitin.  The  origin  of  the 
fat  is  to  be  referred  to  a  retrograde  change  in  the  protoplasmic  ma- 
terial of  the  connective-tissue  cells.  When  this  protoplasm  becomes 
rich  in  carbon  and  hydrogen,  it  is  speedily  converted  into  fat,  which 
makes  its  appearance  in  the  form  of  minute  drops  in  different  portions 
of  the  cell.  As  the  drops  accumulate,  at  the  expense  of  the  cell 
protoplasm  they  gradually  coalesce,  until  there  remains  but  a  thin 
stratum  of  the  protoplasm,  which  forms  the  wall  of  the  vesicle. 
Adipose  tissue  may,  therefore,  be  regarded  as  areolar  tissue,  in  which 
and  at  the  expense  of  some  of  its  elements,  fat  is  stored  for  the 
future  needs  of  the  organism.  A  diminution  of  food,  especially  of  fat 
and  carbohydrates,  is  promptly  followed  by  an  absorption  of  fat  by 
the  blood-vessels  and  by  its  transference  to  the  tissues,  where  it  is 


HISTOLOGY   OF   EPITHELIAL   AND   CONNECTIVE  TISSUES.  39 

either  utilized  for  tissue  construction  or  for  oxidation  purposes.  In 
the  situations  in  which  adipose  tissue  is  found  it  seems,  by  its  chemic 
and  physical  properties,  to  assist  in  the  prevention  of  a  too  rapid 
radiation  of  heat  from  the  body,  to  give  form  and  roundness,  and  to 
diminish  angularities,  etc. 

Retifoim  and  adenoid  tissue  are  also  modifications  of  areolar 
tissue.  The  meshes  of  the  former  contain  but  little  ground  sub- 
stance, its  place  being  taken  by  fluids  ;  the  meshes  of  the  latter  con- 
tain large  numbers  of  lymph  corpuscles. 

Fibrous  Tissue. — This  variety  of  connective  tissue  is  widely  dis- 
tributed throughout  the  body.  It  constitutes  almost  entirely  the  liga- 
ments around  the  joints,  the  tendons  of  the  muscles,  the  membranes 
covering  organs  such  as  the  heart,  liver,  nervous  system,  bones,  etc. 
All  fibrous  tissue,  wherever  found,  can  be  resolved  into  elementary 
bundles,  which  on  microscopic  examination  are  seen  to  consist  of 
delicate,  wavy,  transparent,  homogeneous  fibers,  which  pursue  an 
independent  course,  neither  branching  nor  uniting  with  adjoining 
fibers.  A  small  amount  of  ground  substance  serves  to  hold  them 
together.  Fibrous  tissue  is  tough  and  inextensible,  and  in  conse- 
quence is  admirably  adapted  to  fulfil  various  mechanical  functions 
in  the  body.  It  is,  however,  quite  pliant,  bending  easily  in  all  di- 
rections. When  boiled,  fibrous  tissue  yields  gelatin,  a  derivative  of 
collagen. 

Elastic  Tissue. — The  fibers  of  elastic  tissue  are  usually  associated 
in  varying  proportions  with  the  white  fibrous  tissue ;  but  in  some 
structures — as  the  ligamentum  nuchae,  the  ligamenta  subflava,  the 
middle  coat  of  the  larger  blood-vessels — the  elastic  fibers  are  almost 
the  only  elements  present,  and  give  to  these  structures  a  distinctly 
yellow  appearance.  The  fibers  throughout  their  course  give  off  many 
branches,  which  unite  with  adjoining  branches  to  form  a  more  or 
less  close  network.  As  the  name  implies,  these  fibers  are  highly 
elastic,  and  are  capable  of  being  extended  as  much  as  sixty  per  cent, 
before  breaking. 

Cartilaginous  Tissue. — This  form  of  connective  tissue  differs  from 
the  preceding  varieties  chiefly  in  its  density.  As  a  rule,  it  is  firm 
in  consistency, -though  somewhat  elastic.  It  is  opaque,  bluish-white 
in  color,  though  in  thin  sections  translucent.  All  cartilaginous  tissues 
consist  of  connective-tissue  cells  embedded  in  a  solid  ground  substance. 


40  HUMAN   PHYSIOLOGY. 

According  to  the  amount  and  texture  of  the  ground  substance,  three 
principal  varieties  may  be  distinguished : 

1.  Hyaline  cartilage,  in  which  the  cells,  relatively  few  in  number,  are 
embedded  in  an  abundant  quantity  of  ground  substance.     The  body 
of  the  cells  is  in  many  instances  distinctly  marked  off   from  the 
surrounding  substance  by  concentric  lines  or  fibers,  which  form  a 
capsule  for  the  cell.     Repeated  division  of  the  cell  substance  takes 
place,  until  the  whole  capsule  is  completely  occupied  by  daughter 
cells.      The    ground    substance    is    pervaded    by    minute    channels, 
which  communicate  on  one  hand  with  the  spaces  around  the  cells, 
and  on  the  other  with  lymph-spaces  in  the  connective  tissue  sur- 
rounding   the    cartilage.      By    means    of   these    channels,    nutritive 
fluid    can    permeate    the    entire    structure.      Hyaline    cartilage    is 
found  on  the  ends  of  the  long  bones,  where  it  enters  into  the  for- 
mation of  the  joints;   between  the  ribs  and  sternum,   forming  the 
costal  cartilage,  as  well  as  in  the  nose  and  larynx. 

2.  White  fibre-cartilage,  the  ground  substance  of  which  is  pervaded 
by  white 'fibers,  arranged  in  bundles  or  layers,  between  which  are 
scattered    the   usual    encapsulated    cells.      White    fibro-cartilage    is 
tough,  resistant,  but  flexible,  and  is  found  in  joints  where  strength 
and  fixedness  are  required.     Hence  it  is  present  between  the  ver- 
tebrae, forming  the  intervertebral  discs,  between  the  condyle  of  the 
lower   jaw   and   the   glenoid   fossa,   in   the   knee-joint,    around   the 
margins  of  the  joint  cavities,  etc.     In  these  situations  it  assists  in 
maintaining  the  apposition  of  the  bones,  in  giving  a  certain  degree 
of  mobility  to  the  joints,  and  in  diminishing  the  effects  of  shock 
and  pressure  imparted  to  the  bones. 

3.  Yellow  fibro-cartilage,  the  ground  substance  of  which  is  pervaded 
by  opaque,  yellow  elastic  fibers,  which  form,  by  the  interlacing  of 
their  branches,  a  complicated  network,  in  the  meshes  of  which  are 
to  be  found  the  usual  corpuscles.     As  these  fibers  are  elastic,  they 
impart   to   the   cartilage   a   very   considerable   degree   of   elasticity. 
Yellow  fibro-cartilage  is  well  adapted,  therefore,  for  entering  into 
the  formation  of  the  external  ear,  epiglottis,  Eustachian  tube,  etc. 
— structures  which   require   for  their   functional  activity   a  certain 
degree  of  flexibility  and  elasticity. 

Osseous  Tissue. — Osseous  tissue,  as  distinguished  from  bone,  is  a 
member  of  the  connective-tissue  group,  the  ground  substance  of 
which  is  permeated  with  insoluble  lime  salts,  of  which  the  phos- 


HISTOLOGY   OF  EPITHELIAL  AND  CONNECTIVE  TISSUES.  41 

phate  and  carbonate  are  the  most  abundant.  Immersed  in  dilute 
solutions  of  hydrochloric  acid,  they  can  be  converted  into  soluble 
salts  and  dissolved  out.  The  osseous  matrix  left  behind  is  soft  and 
pliable.  When  boiled,  it  yields  gelatin. 

A  thin,  transverse  section  of  a  decalcified  bone,  when  examined 
microscopically,  reveals  a  number  of  small,  round  or  oval  openings, 
which  represent  transverse  sections  of  canals  which  run  through 
the  bone,  for  the  most  part  in  a  longitudinal  direction,  though  fre- 
quently anastomosing  with  one  another.  These  so-called  Haversian 
canals  in  the  living  state  contain  blood-vessels  and  lymphatics. 

Around  each  Haversian  canal  is  a  series  of  concentric  laminae,  com- 
posed of  white  fibers.  Between  every  two  laminae  are  found  small 
cavities  (lacunae),  from  which  radiate  in  all  directions  small  canals 
(canaliculi),  which  communicate  freely  with  one  another.  The 
Haversian  canals,  with  their  associated  lacunae  and  canaliculi,  form 
a  system  of  intercommunicating  passages,  through  which  lymph  cir- 
culates destined  for  the  nourishment  of  bone.  Each  lacuna  contains 
the  bone  corpuscle,  whicn  bears  a  close  resemblance  to  the  usual 
branched  connective-tissue  corpuscle,  and  whose  function  appears  to 
be  the  maintenance  of  the  nutrition  of  the  bone. 

The  surface  of  every  bone  in  the  recent  state  is  invested  with  a 
fibrous  membrane,  the  periosteum,  except  where  it  is  covered  with 
cartilage.  The  inner  surface  of  this  membrane  is  loose  in  texture, 
and  supports  a  fine  plexus  of  capillary  blood-vessels  and  numerous 
protoplasmic  cells — the  osteoblasts.  As  this  layer  is  directly  con- 
cerned in  the  formation  of  bone,  it  is  spoken  of  as  the  osteogenetic 
layer. 

A  section  of  a  bone  shows  that  it  is  composed  of  two  kinds  of 
tissue — compact  and  cancellated.  The  compact  is  dense,  resembling 
ivory,  and  is  found  on  the  outer  portion  of  the  bone ;  the  cancellated 
is  spongy,  and  appears  to  be  made  up  of  thin,  bony  plates,  which 
intersect  one  another  in  all  directions,  and  is  found  in  greatest 
abundance  in  the  interior  of  the  bones.  The  shaft  of  a  long  bone  is 
hollow.  This  central  cavity,  which  extends  from  one  end  of  the  bone 
to  the  other,  as  well  as  the  interstices  of  the  cancellated  tissue,  is 
filled  in  the  living  state  with  marrow.  The  marrow  or  medulla  is 
composed  of  a  connective-tissue  framework  supporting  blood-vessels. 
In  its  meshes  are  to  be  found  characteristic  bone  cells  or  osteoblasts, 
the  function  of  which  is  supposed  to  be  the  formation  of  bone.  In 
the  long  bones  the  marrow  is  yellow,  from  the  presence  in  the  con- 


42  HUMAN   PHYSIOLOGY. 

nective-tissue  corpuscle  of  fat  globules,  which  arise  through  the 
transformation  of  the  cell  protoplasm.  In  the  cancellated  tissue,  near 
the  extremities  of  the  long  bones,  this  fatty  transformation  does  not 
take  place  to  the  same  extent,  and  the  marrow  appears  red.  The 
cells  of  the  red  marrow  are  believed  to  give  birth  indirectly  to  the 
red  blood-corpuscles. 

Physical   and   Physiologic   Properties   of  Connective  Tissues. — 

Among  the  physical  properties  may  be  mentioned  consistency,  co- 
hesion, and  elasticity.  Their  consistency  varies  from  the  semiliquid 
to  the  solid  state,  and  depends  on  the  quantity  of  water  which  enters 
into  their  composition.  Their  cohesion,  except  in  the  softer  varieties, 
is  very  considerable,  and  offers  great  resistance  to  traction,  pressure, 
torsion,  etc.  In  all  the  movements  of  the  body,  in  the  contraction 
of  muscles,  in  the  performance  of  work,  the  consistence  and  cohesion 
of  these  tissues  play  most  important  roles.  Wherever  the  various 
forms  of  connective  tissue  are  found,  their  chemic  composition  and 
structure  are  in  relation  to  their  functions.  If  traction  be  the  pre- 
ponderating force,  the  structure  becomes  fibrous,  as  in  ligaments  and 
tendons,  and  the  cohesion  greatest  in  the  longitudinal  direction.  If 
pressure  be  exerted  in  all  directions,  as  upon  membranes,  the  fibers 
interlace  and  offer  a  uniform  resistance.  When  pressure  is  exerted 
in  a  definite  direction,  as  on  the  extremities  of  the  long  bones,  the 
tissue  becomes  expanded  and  cancellated.  The  lamellae  of  the  can- 
cellated tissue  arrange  themselves  in  curves  which  correspond  to 
the  direction  of  the  greatest  pressure  or  traction.  Extensibility  is 
not  a  characteristic  feature,  except  in  those  forms  containing  an 
abundance  of  yellow  elastic  fibers.  The  elasticity  is  an  essential 
factor  in  many  physiologic  actions.  It  not  only  opposes  and  limits 
forces  of  traction,  pressure,  torsion,  etc.,  but  on  their  cessation  re- 
turns the  tissues  or  organs  to  their  original  condition.  Elasticity 
thus  assists  in  maintaining  the  natural  form  and  position  of  the 
organs  by  counterbalancing  and  opposing  temporarily  acting  forces. 

The  Skeleton. — The  connective  tissues  in  their  entirety  constitute 
a  framework  which  presents  itself  under  two  aspects:  (i)  As  a 
solid,  bony  skeleton,  situated  in  the  trunk  and  limbs,  affording 
attachment  for  muscles  and  viscera ;  (2)  as  a  fine,  fibrous  skeleton, 
found  everywhere  throughout  the  body,  connecting  the  various  viscera 
and  affording  support  for  the  epithelial,  muscle,  and  nerve  tissues. 


PHYSIOLOGY    OF   THE   SKELETON.  43 

THE    PHYSIOLOGY    OF    THE    SKELETON. 

The  animal  body  is  characterized  by  the  power  of  executing  a  great 
variety  of  movements,  all  of  which  have  reference  to  a  change  of 
relation  of  one  part  of  the  body  to  another,  or  to  a  change  of  position 
of  the  individual  in  space,  as  in  the  various  acts  of  locomotion.  If 
in  the  execution  of  these  movements  the  different  parts  are  applied 
or  directed  to  the  overcoming  of  oppqsing  forces  in  the  environment, 
the  animal  is  said  to  be  doing  work.  In  the  conception  of  the 
animal  body  as  a  machine  for  the  accomplishment  of  work  the 
skeleton,  the  muscle  and  nerve  tissues  constitute  the  three  primary 
mechanisms,  all  of  which  bear  certain  definite  relations  one  to 
another. 

The  Skeleton  is  the  passive  framework,  the  axial  portion  of  which 
(the  vertebral  column,  head,  ribs,  and  sternum)  impart  more  or  less 
fixity  and  rigidity,  while  the  appendicular  portions  (the  bones  of  the 
arms  and  legs)  impart  extreme  mobility.  The  bones  of  the  arms 
and  legs  more  especially  may  be  looked  upon  as  constituting  a  sys- 
tem of  levers,  the  fulcra  of  which,  the  points  of  rest  around  which 
they  move,  lie  in  the  joints. 

That  a  lever  may  be  effective  as  an  instrument  for  the  accomplish- 
ment of  work,  it  must  not  only  be  capable  of  moving  around  its  ful- 
crum, but  it  must  at  the  same  time  be  acted  on  by  two  opposing 
forces,  one  passive,  the  other  active.  In  the  movement  of  the  bony 
levers  of  the  animal  body,  the  passive  forces  are  largely  those  con- 
nected with  the  environment,  e.  g.,  gravity,  cohesion,  friction,  elas- 
ticity, etc.  The  active  forces  by  which  these  latter  are  opposed  and 
overcome  through  the  intermediation  of  the  bony  levers  are  found  in 
the  muscles  attached  to  them.  For  the  execution  of  all  these  move- 
ments, it  is  essential  that  the  relation  of  the  various  portions  of  the 
bony  skeleton  to  one  another  shall  be  such  as  to  permit  of  movement 
while  yet  retaining  close  apposition.  This  is  accomplished  by  the 
mechanical  conditions  which  have  been  evolved  at  the  points  of  union 
of  bones,  and  which  are  technically  known  as  articulations  or  joints. 

A  consideration  of  the  body  movements  involves  an  account  of  (i) 
the  static  conditions,  or  those  states  of  equilibrium  in  which  the  body 
is  at  rest — e.  g.,  standing,  sitting;  (2)  the  dynamic  conditions,  or 
those  states  of  activity  characterized  by  movement — e.  g.,  walking, 
running,  etc.  In  this  connection,  however,  only  those  physical  and  - 


44  HUMAN   PHYSIOLOGY. 

physiologic  peculiarities  of  the  skeleton,  especially  in  its  relation  to 
joints,  will  be  referred  to  which  underlie  and  determine  both  the 
static  and  dynamic  states  of  the  body. 

Structure  of  Joints. — The  structures  entering  into  the  formation 
of  joints  are : 

1.  Bones,  the  articulating  surfaces  of  which  are  often  more  or  less 
expanded,   especially  in   the  case  of  long  bones,   and  at  the   same 
time  variously  modified  and  adapted  to  one  another  in  accordance 
with  the  character  and  extent  of  the  movements  which  there  take 
place. 

2.  Hyaline  cartilage,  which  is  closely  applied  to  the  articulating  end 
of  each  bone.     The  smoothness  of  this  form  of  cartilage  facilitates 
the  movements  of  the  opposing  surfaces,  while  its  elasticity  dimin- 
ishes the   force  of  shocks  and  jars  imparted  to  the  bones  during 
various  muscular  acts.      In  a  number  of  joints,  plates  or  discs  of  white 
fibre-cartilage  are  inserted  between  the  surfaces  of  the  bones. 

3.  A  synovial  membrane,  which  is  attached  to  the  edge  of  the  hyaline 
cartilage  entirely  inclosing  the  cavity  of  the  joint.     This  membrane 
is  composed  largely  of  connective  tissue,  the  inner  surface  of  which 
is  lined  by  endothelial  cells,  which  secrete  a  clear,  colorless,  viscid 
fluid — the  synovia.     This  fluid  not  only  fills  up  the  joint-cavity,  but, 
flowing    over    the    articulating    surfaces,    diminishes    or    prevents 
friction. 

4  Ligaments, — tough,  inelastic  bands,  composed  of  white  fibrous 
tissue, — which  pass  from  bone  to  bone  in  various  directions  on 
the  different  aspects  of  the  joint.  As  white  fibrous  tissue  is  in- 
extensible  but  pliant,  ligaments  assist  in  keeping  the  bones  in 
apposition,  and  prevent  displacement  while  yet  permitting  of  free 
and  easy  movements. 

Classification  of  Joints. — All  joints  may  be  divided,  according  to 
the    extent    and    kind    of    movements    permitted    by    them,    into    (i) 
diarthroses ;    (2)   amphiarthroses ;   (3)   synarthroses. 
i.  Diarthroses. — In  this  division  of  the  joints  are  included  all  those 
which  permit  of  free  movement.     In  the  majority  of  instances  the 
articulating  surfaces   are  mutually  adapted  to   each   other.     If  the 
articulating  surface  of  one  bone  is  convex,  the  opposing  but  cor- 
responding surface  is  concave.     Each  surface,  therefore,  represents 
a  section  of  a  sphere  or  a  cylinder,  which  latter  arises  by  rotation 
of  a  line  around  an   axis  in  space.     According  to  the  number  of 


PHYSIOLOGY   OF  THE   SKELETON.  45 

axes  around  which  the  movements  take  place  all  diarthrodial  joints 
may  be  divided  into : 

i.  Uniaxial  Joints. — In  this  group  the  convex  articulating  surface  is 
a  segment  of  a  cylinder  or  cone,  to  which  the  opposing  surface  more 
or  less  completely  corresponds.  In  such  a  joint  the  single  axis 
of  rotation,  though,  practically  is  not  exactly  at  right  angles  to  the 
long  axis  of  the  bone,  and  hence  the  movements — flexion  and  ex- 
tension— which  take  place  are  not  confined  to  one  plane.  Joints 
of  this  character — e.  g.,the  elbow,  knee,  ankle,  the  phalangeal  joints  of 
the  fingers  and  toes — are,  therefore,  termed  ginglymi,  or  hinge- 
joints.  Owing  to  the  obliquity  of  their  articulating  surfaces,  the 
elbow  and  ankle  are  cochleoid  or  screw -ginglymi.  Inasmuch  as  the 
axes  of  these  joints  on  the  opposite  sides  of  the  body  are  not 
coincident,  the  right  elbow  and  left  ankle  are  right-handed  screws  ; 
the  left  elbow  and  right  ankle,  left-handed  screws.  In  the  knee- 
joint  the  form  and  arrangement  of  the  articulating  surfaces  are 
such  as  to  produce  that  modification  of  a  simple  hinge  known  as  a 
spiral  hinge,  or  helicoid.  As  the  articulating  surfaces  of  the  con- 
dyles  of  the  femur  increase  in  convexity  from  before  backward, 
and  as  the  inner  condyle  is  longer  than  the  outer,  and  therefore, 
represents  a  spiral  surface,  the  line  of  translation  or  the  movement 
of  the  leg  is  also  a  spiral  movement.  During  flexion  of  the  leg 
there  is  a  simultaneous  inward  rotation  around  a  vertical  axis 
passing  through  the  outer  condyle  of  the  femur ;  during  extension 
a  reverse  movement  takes  place.  Moreover,  the  slightly  concave 
articulating  surfaces  of  the  tibia  do  not  revolve  around  a  single 
fixed  transverse  axis,  as  in  the  elbow-joint,  for  during  flexion  they 
slide  backward,  during  extension  forward,  around  a  shifting  axis, 
which  varies  in  position  with  the  point  of  contact. 

In  some  few  instances  the  long  axis  of  the  articulating  surface 
is  parallel  rather  than  transverse  to  the  long  axis,  and  as  the 
movement  then  takes  place  around  a  more  or  less  conic  surface, 
the  joint  is  termed  a  trochoid  or  pulley — e.  g.,  the  odonto-atlantal 
and  the  radio-ulnar.  In  the  former  the  collar  formed  by  the  atlas 
and  its  transverse  ligament  rotates  around  the  vertical  odontoid 
process  of  the  axis.  In  the  latter  the  head  of  the  radius  revolves 
around  its  own  long  axis  upon  the  ulna,  giving  rise  to  the  move- 
ments of  pronation  and  supination  of  the  hand.  The  axis  around 
.  which  these  two  movements  take  place  is  continued  through  the 
head  of  the  radius  to  the  styloid  process  of  the  ulna. 


46  HUMAN   PHYSIOLOGY. 

2.  Biaxial   Joints. — In   this   group   the    articulating   surfaces    are   un- 
equally  curved,    though    intersecting   each    other.      When   the    sur- 
faces lie  in  the  same  direction,  the  joint  is  termed  an  ovoid  joint — 
e.  g.}  the   radio-carpal   and   the   atlanto-occipital.      As   the   axes   of 
these  surfaces  are  vertical  to  each  other,  the  movements  permitted 
by  the  former  joint  are  flexion,   extension,  adduction,   and  abduc- 
tion, combined  with  a  slight  amount  of  circumduction ;  the  latter 
joint  permits  of  flexion  and  extension  of  the  head,  with  inclination 
to  either  side.     When  the  surfaces  do  not  take  the  same  direction, 
the   joint,    from    its    resemblance   to   the    surfaces    of   a    saddle,    is 
termed  a  saddle-joint — e.  g.,  the  trapezio-metacarpal.     The  move- 
ments permitted  by  this  joint  are  also  flexion,  extension,  adduction, 
abduction,  and  circumduction. 

3.  Polyaxial  Joints. — In  this  group  the  convex  articulating  surface  is 
a  segment  of  a  sphere,  which  is  received  by  a  socket  formed  by 
the    opposing    articulating    surface.      In    such    a    joint,    termed    an 
enarthrodial    or    ball-and-socket    joint, — e.    g.,    the    shoulder-joint, 
hip-joint, — the   distal   bone   revolves    around   an    indefinite   number 
of  axes,  all  of  which  intersect  one  another  at  the  center  of  rota- 
tion.    For  simplicity,  however,  the  movement  may  be  described  as 
taking  place  around  axes  in  the  three  ordinal  planes — viz.,  a  trans- 
verse,   a    sagittal,    and    a    vertical    axis.      The    movements    around 
the    transverse    axis    are    termed    flexion    and    extension ;    around 
the    sagittal    axis,    adduction    and    abduction ;    around   the    vertical 
axis,  rotation.     When  the  bone  revolves  around  the  surface  of  an 
imaginary  cone,  the  apex  of  which   is  the  center   of  rotation   and 
the  base  the  curve  described  by  the  hand,  the  movement  is  termed 
circumduction. 

2.  Amphiarthroses. — In    this    division    are    included    all    those    joints 
which  permit  of  but  slight  movement — e.  g.,  the  intervertebral,  the 
interpubic,   and  the  sacro-iliac  joints.     The  surfaces  of  the  oppos- 
ing bones  are  united  and  held  in  position  largely  by  the  intervention 
of    a    firm,    elastic    disc    of    nbro-cartilage.       Each    joint    is    also 
strengthened  by  ligaments. 

3.  Synarthroses. — In    this    division    are    included    all    those    joints    in 
which   the   opposing   surfaces   of  the   bones   are   immovably   united, 
and  hence  do  not  permit  of  any  movement — e.  g.,  the  joints  between 
the  bones  of  the  skull. 

The  Vertebral  Column. — In  all  static  and  dynamic  states  of  the 
body  the  vertebral  column  plays  a  most  essential  role.     Situated  in 


PHYSIOLOGY    OF   THE    SKELETON.  4  < 

the  middle  of  the  back  of  the  trunk,  it  forms  the  foundation  of  the 
entire  skeleton.  It  is  composed  of  a  series  of  superimposed  bones, 
termed  vertebrae,  which  increase  in  size  from  above  downward  as 
far  as  the  brim  of  the  pelvic  cavity.  Superiorly,  it  supports  the  skull ; 
laterally,  it  affords  attachment  for  the  ribs,  which  in  turn  support 
the  weight  of  the  upper  extremities  ;  below,  it  rests  upon  the  pelvic 
bones,  which  transmit  the  weight  of  the  body  to  the  inferior  extremi- 
ties. The  bodies  of  the  vertebrae  are  united  one  to  another  by 
tough  elastic  discs  of  nbro-cartilage,  which,  collectively,  constitute 
about  one  quarter  of  the  length  of  the  vertebral  column.  The  vertebrae 
are  held  together  by  ligaments  situated  on  the  anterior  and  posterior 
surfaces  of  their  bodies,  and  by  short,  elastic  ligaments  between  the 
neural  arches  and  processes.  These  structures  combine  to  render 
the  vertebral  column  elastic  and  flexible,  and  enable  it  to  resist  and 
diminish  the  force  of  shocks  communicated  to  it. 

The  amphiarthrodial  character  of  the  intervertebral  joint  endows 
the  entire  column  with  certain  forms  of  movement  which  are  neces- 
sary to  the  performance  of  many  body  activities.  While  the  range 
of  movement  between  any  two  vertebrae  is  slight,  the  sum  total  of 
movement  of  the  entire  series  of  vertebrae  is  considerable.  In 
different  regions  of  the  column  the  character,  as  well  as  the  range 
of  movement,  varies  in  accordance  with  the  form  of  the  vertebrae  and 
the  inclination  of  their  articular  processes.  In  the  cervical  and  lum- 
bar regions  extension  and  flexion  are  freely  permitted,  though  the 
former  is  greater  in  the  cervical,  the  latter  in  the  lumbar  region, 
especially  between  the  fourth  and  fifth  vertebrae.  Lateral  flexion  takes 
place  in  all  portions  of  the  column,  but  is  particularly  marked  in  the 
cervical  region.  A  rotatory  movement  of  the  column  as  a  whole 
takes  place  through  an  angle  of  about  twenty-eight  degrees.  This  is 
most  evident  in  the  lower  cervical  and  dorsal  regions. 

The  skeleton  may,  therefore  be  regarded  as  a  highly  developed 
framework,  which  determines  not  only  the  form  of  the  body,  and 
affords  support  and  protection  to  the  various  softer  organs  and 
tissues,  but  also,  through  the  mobility  of  its  joints,  permits  of  a 
great  variety  of  complicated  movements. 


48  HUMAN   PHYSIOLOGY. 

GENERAL    PHYSIOLOGY    OF    MUSCLE 
TISSUE. 

The  muscle  tissue,  which  closely  invests  the  bones  of  the  body, 
and  which  is  familiar  to  all  as  the  flesh  of  animals,  is  the  immediate 
cause  of  the  active  movements  of  the  body.  This  tissue  is  grouped 
in  masses  of  varying  size  and  shape,  which  are  technically  known  as 
muscles.  The  majority  of  the  muscles  of  the  body  are  connected 
with  the  bones  of  the  skeleton  in  such  a  manner  that,  by  an  altera- 
tion in  their  form,  they  can  change  not  only  the  position  of  the  bones 
with  reference  to  one  another,  but  can  also  change  the  individual's 
relation  to  surrounding  objects.  They  are,  therefore,  the  active 
organs  of  both  motion  and  locomotion,  in  contradistinction  to  the 
bones  and  joints,  which  are  but  passive  agents  in  the  performance 
of  the  corresponding  movements.  In  addition  to  the  muscle  masses 
which  are  attached  to  the  skeleton,  there  are  also  other  collections 
of  muscle  tissue  surrounding  cavities  such  as  the  stomach,  intestine, 
blood-vessels,  etc.,  which  impart  to  their  walls  motility,  and  so  influ- 
ence the  passage  of  a  material  through  them. 

Muscles  produce  movement  of  the  structures  to  which  they  are 
attached  by  the  property  with  which  they  are  endowed  of  changing 
their  shape,  shortening  or  contracting  under  the  influence  of  a  stimu- 
lus transmitted  to  them  from  the  nervous  system.  Muscles  are  there- 
fore divided  into  : 

1.  Voluntary  muscles,  comprising  those  whose  activity  is  called  forth 
by  stimuli  of  the  nerves  as  the  result  of  an  act  or  effort  of  volition. 

2.  Involuntary  muscles,  comprising  those   whose   activity  is   entirely 
independent  of  the  volition. 

The  voluntary  muscles  are  also  known  from  their  attachment  to 
the  skeleton  as  skeletal,  and  from  their  microscopic  appearance  as 
striped  muscles.  The  involuntary  muscles,  from  their  relation  to 
the  viscera  of  the  body,  are  known  also  as  visceral,  and  from  their 
microscopic  appearance  as  plain  or  smooth  muscles. 

General  Structure  of  Muscles. — All  skeletal  muscles  consist  of 
a  central  fleshy  portion,  the  body  or  belly,  which  is  provided  at  either 
extremity  with  a  tendon  in  the  form  of  a  cord  or  membrane  by  which 
it  is  attached  to  the  bones.  The  body  is  the  contractile  region,  the 
source  of  activity ;  the  tendon  is  a  passive  region,  and  merely  trans- 
mits the  activity  to  the  bones. 


PHYSIOLOGY   OF   MUSCLE  TISSUE.  49 

A  skeletal  muscle  is  a  complex  organ  consisting  of  muscular  fibers, 
connective  tissue,  blood-vessels,  and  lymphatics.  The  general  body 
of  the  muscle  is  surrounded  by  a  dense  layer  of  connective  tissue, 
the  epimysium,  which  blends  with  and  partly  forms  the  tendon ;  from 
its  inner  surface  septa  of  connective  tissue  pass  inward  and  group 
the  muscle-fibers  into  larger  and  smaller  bundles,  termed  fasciculi. 
.The  fasciculi,  invested  by  this  special  sheath,  the  perimysium,  are 
irregular  in  shape,  and  vary  considerably  in  size.  The  fibers  of  the 
fasciculi  are  separated  from  one  another  and  supported  by  a  deli- 
cate connective  tissue,  the  endomysium.  The  connective  tissue  thus 
surrounding  and  penetrating  the  muscle  binds  its  fibers  into  a  distinct 
organ,  and  affords  support  to  blood-vessels,  nerves,  and  lymphatics. 
The  muscle  fibers  are  arranged  parallel  to  one  another,  and  their 
direction  is  that  of  the  long  axis  of  the  muscle.  In  length  they  vary 
from  thirty  to  forty  millimeters,  and  in  diameter  from  twenty  to 
thirty  micromillimeters. 

The  vascular  supply  to  the  muscles  is  very  great,  and  the  disposi- 
tion of  the  capillary  vessels,  with  reference  to  muscle-fiber,  is  very 
characteristic.  The  arterial  vessels,  after  entering  the  muscle,  are 
supported  by  the  perimysium ;  in  this  situation  they  give  off  short, 
transverse  branches,  which  immediately  break  up  into  a  capillary 
network  of  rectangular  shape,  within  which  the  muscle-fibers  are 
contained.  The  muscle-fiber  in  intimate  relation  with  the  capillary 
is  bathed  with  lymph  derive^  from  it.  Its  contractile  substance, 
however,  is  separated  from  the  lymph  by  its  own  investing  membrane, 
through  which  all  interchange  of  nutritive  and  waste  materials  must 
take  place.  Lymphatics  are  present  in  muscle,  but  are  confined  to 
the  connective  tissue,  in  the  spaces  of  which  they  have  their  origin. 

The  nerves  which  carry  the  stimuli  to  a  muscle  enter  near  its 
geometric  center.  Many  of  the  fibers  pass  directly  to  the  muscle- 
fibers  with  which  they  are  connected ;  others  are  distributed  to  blood- 
vessels. Every  muscle-fiber  is  supplied  with  a  special  nerve-fiber, 
except  in  those  instances  where  the  nerve  trunks  entering  a  muscle 
do  not  contain  so  many  fibers  as  the  muscle.  In  such  cases  the  nerve- 
fibers  divide,  until  the  number  of  branches  equals  the  number  of 
muscle-fibers.  The  individual  muscle-fiber  is  penetrated  near  its 
center  by  the  nerve,  the  ends  being  practically  free  from  nerve 
influence.  The  stimulus  that  comes  to  the  muscle  fiber  acts  primarily 
upon  its  center,  and  then  travels  in  both  directions  to  the  ends. 
5 


50  HUMAN   PHYSIOLOGY. 

Histology  of  the  Skeletal  Muscle-fiber. — A  muscle-fiber  consists 
of  a  transparent  elastic  membrane,  the  sarcolemma,  in  which  is 
contained  the  true  muscle  element.  Examined  microscopically,  the 
fiber  presents  a  series  of  alternate  dim  and  bright  bands,  giving  to 
it  a  striated  appearance. 

When  the  bright  band  is  examined  with  high  magnifying  powers, 
a  fine,  dark  line  is  seen  crossing  it  transversely.  It  was  supposed  b% 
Krause  to  be  the  optic  expression  of  a  membrane  which  divides  the 
cavity  of  the  sarcolemma  into  a  series  of  compartments,  each  of 
which  contains  a  dim  band  of  sarcous  or  muscle  substance,  bounded 
at  either  extremity  with  the  half  of  a  bright  band.  This  membrane 
has  since  been  resolved  into  a  row  of  granules. 

The  muscle-fiber  also  exhibits  a  longitudinal  striation,  indicating 
that  it  is  composed  of  fibrillae,  placed  side  by  side  and  embedded  in 
some  interfibrillar  substance,  to  which  the  name  sarcoplasm  has  been 
given.  The  fibrillae,  which  are  arranged  longitudinally  to  the  long 
axis  of  the  fiber,  are  grouped  by  the  intervening  material  into 
bundles  of  varying  size,  the  muscle  columns.  The  fibrillae  which  ex- 
tend throughout  the  length  of  the  fiber  are  not  of  uniform  thickness, 
but  present  at  regular  intervals  well-marked  constrictions. 

In  the  region  of  the  dim  band  the  fibrilla  presents  itself  in  the  form 
of  a  homogeneous  prismatic  rod,  termed  sarcostyle,  separated  from 
neighboring  rods  by  a  slight  amount  of  sarcoplasm.  Between  two 
successive  rods  is  found  a  dark  granule,  united  by  a  thin  band  of 
similar  material  to  the  ends  of  the  rods.  The  transverse  row  of 
granules  corresponds  to  Krause's  membrane., 

In  the  region  of  the  granules  there  is  a  diminution  of  the  sarcous 
substance,  but  an  increase  in  the  amount  of  sarcoplasm,  and  as  the 
latter  is  more  transparent  than  the  former,  the  fiber  presents  at  this 
point  a  conspicuous  bright  band.  Rollet  considers  the  sarcostyles 
to  be  preexistent,  not  the  result  of  post-mortem  or  chemic  changes, 
and  the  seat  of  the  contractile  elements.  The  sarcoplasm  is  a  passive 
material  similar  in  its  properties  to  protoplasm. 

Briicke  has  shown  that  when  the  muscle-fiber  is  examined  under 
crossed  Nicol  prisms  the  dim  band  appears  bright  and  the  bright 
band  appears  dim  against  a  dark  background,  indicating  that  the 
former  is  doubly  refractile,  or  anisotropic,  the  latter  singly  refractile, 
or  isotropic.  The  fiber,  therefore,  appears  to  be  composed  of  alternate 
discs  of  anisotropic  and  isotropic  substance. 


PHYSIOLOGY   OF    MUSCLE   TISSUE.  ^1 

Structure  of  Non-striated  Muscle-fiber. — As  the  name  implies, 
the  involuntary  fiber  is  non-striated,  being  apparently  uniform  and 
homogeneous  in  appearance.  When  isolated,  the  fiber  presents  itself 
in  the  form  of  an  elongated  fusiform  cell,  varying  from  T^  to  ^J^  of 
an  inch  in  length.  In  some  animals  the  fiber  exhibits  a  longitudinal 
striation,  as  if  it  were  composed  of  fibers.  The  cell  is  surrounded  by 
a  thin,  elastic  membrane,  and  contains  a  distinct  oval  nucleus.  The 
fibers  are  usually  arranged  in  bundles  and  lamellae,  and  held  together 
by  a  cement  substance  and  connective  tissue.  This  non-striated 
muscle  tissue  is  found  in  the  muscularis  mucosse  of  the  alimentary 
canal  as  well  as  in  the  muscular  walls  of  the  stomach  and  intestines, 
in  the  posterior  part  of  the  trachea,  in  the  bronchial  tubes,  in  the 
walls  of  the  blood-vessels,  and  in  many  other  situations. 

Chemic  Composition  of  Muscle. — The  chemic  composition  of 
muscle  is  imperfectly  understood,  owing  to  the  fact  that  some  of  its 
constituents  undergo  a  spontaneous  coagulation  after  death,  and  that 
the  chemic  methods  employed  also  tend  to  alter  its  normal  composi- 
tion. When  fresh  muscle  is  freed  from  fat  and  connective  tissue, 
frozen,  rubbed  up  in  a  mortar,  and  expressed  through  linen,  a  slightly 
yellow,  syrupy,  alkaline,  or  neutral  fluid  is  obtained,  known  as 
muscle  plasma.  This  fluid  at  normal  temperature  coagulates  spon-. 
taneously,  and  resembles  in  many  respects  the  coagulation  of  blood 
plasma.  The  coagulum  subsequently  contracts  and  squeezes  out 
an  acid  muscle  serum.  The  coagulated  mass  is  termed  myosin. 
This  proteid  belongs  to  the  class  of  globulins.  Inasmuch  as  it  is  not 
present  in  living  muscle,  and  makes  its  appearance  only  in  the  as  yet 
living  muscle  plasma,  it  is  probable  that  it  is  derived  from  some  pre- 
existing substance,  which  is  supposed  to  be  myosinogen.  Myosin  is 
digested  by  pepsin  and  trypsin.  According  to  Halliburton,  muscle 
plasma  contains  the  following  proteid  bodies :  Myosinogen,  para- 
myosinogen,  albumin,  myoalbumose;  all  of  which  differ  in  chemic 
composition  and  respond  to  various  chemic  and  physical  reagents. 

Ferment  bodies,  such  as  pepsin  and  diastase ;  non-nitrogenized 
bodies,  such  as  glycogen,  lactic  and  sarcolactic  acids,  fatty  bodies,  and 
inosite ;  nitrogenized  extractives — e.  g.,  urea,  uric  acid,  kreatinin,  as 
well  as  inorganic  salts,  have  been  obtained  from  the  muscle  serum. 

Metabolism  in  Muscles. — The  chemic  changes  which  underlie  the 
transformation  of  energy  in  living  muscles  are  very  active  and 
complex. 


52  HUMAN    PHYSIOLOGY. 

As  shown  by  an  analysis  of  the  blood  flowing  to  and  from  the 
resting  muscle,  it  has,  while  passing  through  the  capillaries,  lost 
oxygen  and  gained  carbon  dioxid.  The  amount  of  oxygen  absorbed 
by  the  muscle  (nine  per  cent.)  is  greater  than  the  amount  of  CO2 
given  off  (6.7  per  cent.).  There  is  no  parallelism  between  these  two 
processes,  as  CO2  will  be  given  off  in  the  absence  of  oxygen,  or  in  an 
atmosphere  of  nitrogen. 

In  the  active  or  contracting  muscle  both  the  absorption  of  oxygen 
and  the  production  of  CO2  are  largely  increased,  but  the  ratio  existing 
between  them  differs  considerably  from  that  of  the  resting  muscle, 
for  the  quantity  of  oxygen  absorbed  amounts  to  11.26  per  cent., 
the  quantity  of  CO2  to  10.8  per  cent.  (Ludwig).  Moreover,  in 
a  tetanized  muscle  the  quantity  of  CO2  given  off  may  be  largely  in 
excess  of  the  oxygen  absorbed.  From  these  facts  it  is  evident  that 
the  energy  of  the  contraction  does  not  depend  upon  the  direct  oxida- 
tion of  certain  substances,  but  upon  the  decomposition  of  some  un- 
stable compound  of  high  potential  energy,  rich  in  carbon  and  oxygen. 
When  the  muscle  is  active,  its  tissue  changes  from  a  neutral  to  an 
acid  reaction,  from  the  development  of  sarcolactic  and  possibly  phos- 
phoric acids.  The  amount  of  glycogen  present  in  muscle  (0.43  per 
cent.)  diminishes,  but  muscles  wanting  in  glycogen,  nevertheless, 
retain  their  power  of  contraction.  Water  is  absorbed.  The  amount 
of  urea  is  not  materially  increased  by  muscular  activity,  unless  it  is 
excessive  and  prolonged,  and  then  only  in  the  absence  of  a  suffi- 
cient quantity  of  non-nitrogenized  material.  Coincident  with  muscle 
contraction,  the  blood-vessels  become  widely  dilated,  leading  to  a 
large  increase  in  the  blood-supply  and  a  rapid .  removal  of  products 
of  decomposition. 

Rigor  Mortis. — A  short  time  after  death  the  muscles  pass  into  a 
condition  of  extreme  rigidity  or  contraction,  which  lasts  from  one  to 
five  days.  In  this  state  they  offer  great  resistance  to  extension,  their 
tonicity  disappears,  their  cohesion  diminishes,  their  irritability  ceases. 
The  time  of  the  appearance  of  this  post-mortem  or  cadaveric  rigidity 
varies  from  a  quarter  of  an'  hour  to  seven  hours.  .  Its  onset  and 
duration  are  influenced  by  the  condition  of  the  muscular  irritability 
at  the  time  of  death.  When  the  irritability  is  impaired  from  any 
cause,  such  as  disease  or  defective  blood-supply,  the  rigidity  appears 
promptly,  but  is  of  short  duration.  After  death  from  acute  diseases, 
it  is  apt  to  be  delayed,  but  to  continue  for  a  longer  period. 


PHYSIOLOGY   OF   MUSCLE  TISSUE.  53 

The  rigidity  appears  first  in  the  muscles  of  the  lower  jaw  and 
neck ;  next  in  the  muscles  of  the  abdomen  and  upper  extremities ; 
finally  in  the  trunk  and  lower  extremities.  It  disappears  in  practi- 
caHy  the  same  order. 

Chemic  changes  of  a  marked  character  accompany  this  rigidity. 
The  muscle  becomes  acid  in  reaction  from  the  development  of  sarco- 
lactic  acid ;  it  gives  off  a  large  quantity  of  carbonic  acid,  and  is 
shortened  and  diminished  in  volume. 

The  immediate  cause  of  the  rigidity  appears  to  be  a  coagulation  of 
the  myosinogen  within  the  sarcolemma,  with  the  subsequent  formation 
of  myosin  and  muscle  serum.  In  the  early  stages  of  coagulation 
restitution  is  possible  by  the  circulation  of  arterial  blood  through 
the  vessels.  The  final  disappearance  of  this  contraction  is  due  to 
the  action  of  acids  dissolving  the  myosin,  and  possibly  to  putrefactive 
changes. 

Source  of  Muscular  Energy. — According  to  most  experimenters,  it 
is  certain  that  "normal  muscle  activity  is  not  dependent  on  the  meta- 
bolism of  nitrogenous  materials,  inasmuch  as  its  chief  end  product, 
urea,  is  not  increased.  The  marked  production  of  CO2  points  to  the 
combustion  of  some  non-nitrogenous  matter, — e.  g.,  glycogen, — espe- 
cially as  this  substance  disappears  during  muscular  activity.  Muscles 
wanting  in  glycogen  are,  nevertheless,  capable  of  contracting  for 
some  time.  Moreover,  there  is  no  proof  of  the  direct  combustion  of 
glycogen  or  any  other  carbohydrate.  It  has  been  suggested  by  Her- 
mann that  the  energy  of  a  muscular  contraction  may  be  due  to  the 
splitting  and  subsequent  re-formation  of  a  complex  body  belonging 
neither  to  the  carbohydrates  nor  to  the  fats,  but  to  the  albumins. 
To  this  body  the  term  inogen  has  been  applied.  This  complex 
molecule,  the  product  of  the  metabolic  activity  of  the  muscle  cell, 
in  undergoing  decomposition  would  yield  CO2,  sarcolactic  acid,  and 
a  proteid  residue  resembling  myosin.  With  the  cessation  of  the 
contraction,  the  muscle  protoplasm  recombines  the  proteid  residue 
with  oxygen,  carbohydrates,  and  fats,  and  again  forms  inogen. 

The  phenomena  of  rigor  mortis  support  such  a  view.  At  the  mo- 
ment of  this  contraction  the  muscle  gives  off  CO2  in  large  amount,  the 
muscle  becomes  acid,  and  myosin  is  formed.  There  is  thus  a  close 
analogy  between  the  two  processes  ;  in  other  words,  a  contraction  is 
a  partial  death  of  the  muscle.  As  to  what  becomes  of  the  myosin 
formed  during  a  contraction,  nothing  is  known.  It  may  be  used  in 
the  formation  of  new  inogen. 


54  HUMAN    PHYSIOLOGY. 

The  Physical  Properties  of  Muscle  Tissue. — The  consistency  of 
muscle  tissue  varies  considerably,  according  to  the  different  states  of 
the  muscle.  In  a  state  of  tension  it  is  hard  and  resistant ;  when  free 
from  tension,  it  is  soft  and  fluctuating,  whether  the  muscle  is  con- 
tracting or  resting.  Tension  alone  produces  hardness.  The  cohesion 
of  muscle  tissue  is  less  than  that  of  connective  tissue,  and  is  broken 
more  readily.  Cohesion  resists  traction  and  pressure,  and  lasts  as 
long  as  irritability  remains. 

The  elasticity  of  a  muscle,  though  not  great  is  almost  perfect. 
After  being  extended  by  a  weight,  it  returns  to  its  natural  form.  The 
limit  of  elasticity,  however,  is  soon  passed.  A  weight  of  50  or  100 
grams  will  overcome  the  elasticity  so  that  it  will  not  return  to  its 
natural  length.  In  inorganic  bodies  the  extension  is  directly  propor- 
tional to  the  extending  weight,  and  the  line  of  extension  is  straight. 
With  muscles,  the  extension  is  not  proportional  to  the  weight. 
While  at  first  it  is  marked,  the  elongation  diminishes  as  the  weight 
increases  by  equal  increments,  so  that  the  line  of  extension  becomes 
a  curve.  In  other  words,  the  elasticity  of  a  passive  muscle  aug- 
ments with  increased  extension.  On  the  contrary,  the  elasticity  of 
an  active  is  less  than  that  of  a  passive  muscle,  for  it  is  elongated 
more  by  the  same  weight,  as  shown  by  experiment. 

Tonicity  is  a  property  of  all  muscles  in  the  body,  in  consequence 
of  being  normally  stretched  to  a  slight  extent  beyond  their  natural 
length.  This  may  be  due  to  the  action  of  antagonistic  muscles,  or  to 
the  elasticity  of  the  parts  of  the  skeleton  to  which  they,  are  attached. 
This  is  shown  by  the  shortening  of  the  muscle  which  takes  place  when 
it  is  divided.  Muscular  tonus  plays  an  important  role  in  muscular 
contraction.  Being  always  on  the  stretch,  the  muscle  loses  no  time 
in  acquiring  that  degree  of  tension  necessary  to  its  immediate  action 
on  the  bones.  Again,  the  working  power  of  a  muscle  is  increased  by 
the  presence  of  some  resistance  to  the  act  of  contraction.  According 
to  Marey,  the  amount  of  work  is  considerably  increased  when  the 
muscular  energy  is  transmitted  by  an  elastic  body  to  the  mass  to  be 
moved,  while  at  the  same  time,  the  shock  of  the  contraction  is 
lessened.  The  position  of  a  passive  limb  is  the  resultant  also  of  the 
elastic  tension  of  antagonistic  groups  of  muscles. 

Muscle  excitability  or  contractility  are  terms  employed  to  denote 
that  property  of  muscle  tissue  in  virtue  of  which  it  contracts  or 
shortens  in  response  to  various  excitants  or  stimuli.  Though  usually 


PHYSIOLOGY  OF   MUSCLE  TISSUE.  55 

associated  with  the  activity  of  the  nervous  system,  it  is  nevertheless, 
an  independent  endowment,  and  persists  after  all  nervous  connections 
are  destroyed.  If  the  nerve  terminals  be  destroyed,  as  they  can  be 
by  the  introduction  of  curara  into  the  system,  the  muscles  become 
completely  relaxed  and  quiescent.  The  strongest  stimuli  applied  to  the 
nerves  fail  to  produce  a  contraction.  Various  external  stimuli  applied 
directly  to  the  muscle  substance  produce  at  once  the  characteristic 
contraction.  The  excitability  of  muscle  is  therefore  an  inherent 
property,  dependent  on  its  nutrition,  and  persisting  as  long  as  it  is 
supplied  with  proper  nutritive  materials  and  surrounded  by  those 
external  conditions  which  maintain  its  chemic  or  physical  integrity. 

Muscle  Contractions. — All  muscle  contractions  occurring  in  the 
body  under  normal  physiologic  conditions  are  either  voluntary,  caused 
by  a  volitional  effort  and  the  transmission  of  a  nerve  impulse  from 
the  brain  through  the  spinal  cord  and  nerves  to  the. muscles,  or  reftex, 
caused  by  a  peripheral  stimulation  and  the  transmission  of  a  nerve 
impulse  to  the  spinal  cord,  to  be  reflected  outward  through  the  same 
nerves  to  the  muscles.  In  either  case  the  resulting  contraction  is 
essentially  the  same.  The  normal  or  physiologic  stimulus  which 
provokes  the  muscular  contraction  is  a  nerve  impulse  the  nature  of 
which  is  unknown,  but  is  perhaps  allied  to  a  molecular  disturbance. 
After  removal  from  the  body,  muscles  remain  in  a  state  of  rest, 
inasmuch  as  they  possess  no  spontaneity  of  action.  Though  consist- 
ing of  a  highly  irritable  tissue,  they  can  not  pass  from  the  passive 
to  the  active  state  except  upon  the  application  of  some  form  ojE  stimu- 
lation. 

The  stimuli  which  are  capable  of  calling  forth  a  contraction  may 
be  divided  into — 

1.  Mechanical. 

2.  Chemic. 

3.  Physical. 

4.  Electric. 

Every  mechanical  stimulus  of  a  muscle, — e.  g.,  pick,  cut,  or  tap/ — 
provided  it  has  sufficient  intensity,  and  is  repeated  with  sufficient 
rapidity,  will  cause  not  only  a  single  contraction,  but  a  series  of 
contractions. 

All  chemic  agents  which  impair  the  chemic  composition  of  the 
muscle  with  sufficient  rapidity — -e.  g.,  hydrochloric  acid,  acetic  and 
oxalic  acids,  distilled  water  injected  into  the  vessels,  etc. — act  as 


56  HUMAN   PHYSIOLOGY. 

stimuli,  and  produce  single  and  multiple  contractions.  Physical 
agents,  as  heat  and  electricity,  also  act  as  stimuli.  A  muscle  heated 
rapidly  to  30°  C.  contracts  vigorously,  and  reaches  its  maximum  at 
45"  C.  Of  all  forms  of  stimuli,  the  electric  is  the  most  generally 
used.  Two  forms  are  used — the  induced  current  and  the  make-and- 
break  of  a  constant  current. 

Changes  in  a  Muscle  During  Contraction. — When  a  muscle  is 
stimulated,  either  indirectly  through  the  nerve  or  directly  by  any 
external  agent,  it  undergoes  a  series  of  changes,  which  relate  to  its 
form,  volume,  optic,  physical,  chemic,  and  electric  properties.  These 
changes,  in  their  totality,  constitute  the  muscular  contraction. 

1.  Form. — The   most   obvious   change   is   that   of   form.      The   fibers 
become  shorter  in  their  longitudinal  and  wider  in  their  transverse 
diameters,  and  the  muscle  as  a  whole  becomes  shorter  and  thicker. 
The  degree  of  shortening  may  amount  to  thirty  per  cent,   of  the 
original  length. 

2.  Volume. — The  increase  in  transverse  diameter  does  not  fully  com- 
pensate for  the  diminution  in  length,   for  there  is  at  the  moment 
of    contraction    a    slight    shrinkage    in    volume,    which    has    been 
attributed  to  a  compression  of  air  in  its  interstices. 

3.  Optic    Changes. — If    a    muscle-fiber    be    examined    microscopically 
during  its  contraction,  it  will  be  observed  that  when  the  contrac- 
tion wave  begins,  both  bright  and  dim  bands  diminish  in  height  and 
become    broader,    though    this    change    is    more    noticeable    in    the 
region  of  the  bright  band.     This  Englemann  attributes  to  a  passage 
of  fluid  material  from  the  bright  into  the  dim  band.     At  the  time 
of  relaxation  there  is  a  return  of  this  material,  and  the  fiber  as- 
sumes  its   original   shape   and   volume.      As   the   contraction   wave 
reaches  its  maximum,  the  optic  properties  of  both  the  isotropic  and 
anisotropic    bands    change.       The    former,    which    was    originally 
clear,  now  becomes  darker  and  less  transparent,  until  at  the  crest 
of  the  wave  it  assumes  the  appearance  of  a  distinct  dark  band.    The 
latter,   the    anisotropic,   which   was    originally    dim,    now    becomes, 
in  comparison,   clear  and  light.     This   change  in  optic  appearance 
is  due  to  an  increase  in  refrangibility  of  the  isotropic  and  a  de- 
crease in  the  anisotropic  bands  coincident  with  the  passage  of  fluid 
from   the   former  into  the   latter.     There   is   at   the   height   of  the 
contraction  a  complete  reversal  in  the  positions  of  the  striations. 
At  a  certain  stage  between  the  beginning  and  the  crest  of  the  wave 
these  is  an  intermediate  point,  at  which  the  striae  almost  entirely 


PHYSIOLOGY  OF   MUSCLE  TISSUE.  57 

disappear,  giving  to  the  fiber  an  appearance  of  homogeneity.  There 
is,  however,  no  change  in  refractive  power,  as  shown  by  the 
polarizing  apparatus.  After  the  contraction  wave  has  reached  the 
stage  of  greatest  intensity,  there  is  a  reversal  of  the  foregoing 
phenomena,  and  the  fiber  returns  to  its  original  condition,  which 
is  one  of  relaxation. 

Physical  Changes. — The  extensibility  of  muscle  is  increased  during 
the  contraction,  the  same  weight  elongating  the  fibers  to  a  greater 
extent  than  during  rest.  The  elasticity,  or  its  power  of  returning  to 
its  original  form  is  correspondingly  diminished. 

Chemic  Changes. — The  metabolism  of  muscle  during  the  contraction 
is  very  active.  There  is  an  increase  in  the  production  of  carbon 
dioxid  and  in  the  absorption  of  oxygen.  The  muscle  changes  from 
an  alkaline  or  neutral  to  an  acid  reaction,  from  the  development 
of  sarcolactic  acid.  The  muscle  also  becomes  warmer.  The  electric 
changes  will  be  treated  of  in  connection  with  nerves. 

Transmission  of  the  Contraction  Wave. — Normally,  when  a 
muscle  is  stimulated  by  the  nerve  impulse,  the  shortening  and 
thickening  of  the  fibers  begin  at  the  end  organ  and  travel  in  opposite 
directions  to  the  ends  of  the  muscle.  This  change  propagates  itself  in 
a  wave-like  manner,  and  has  been  termed  the  contraction  wave.  If 
a  stimulus  be  applied  directly  to  the  end  of  a  long  muscle,  the  con- 
traction wave  passes  along  its  entire  length  to  the  opposite  extremity, 
in  virtue  of  the  conductivity  of  muscular  tissue.  The  rapidity  of  the 
propagation  varies  in  different  animals — in  the  frog,  from  three  to 
four  meters  a  second,  in  man,  from  ten  to  thirteen  meters.  The 
length  of  the  wave  varies  from  200  to  400  millimeters. 

Graphic  Record  of  a  Muscle  Contraction. — The  changes  in  the 
form  of  a  muscle  during  contraction  and  relaxation  have  been  care- 
fully studied  by  recording  the  muscle  movement  by  means  of  an 
attached  lever,  the  end  of  which  is  allowed  to  rest  upon  a  moving 
surface.  The  time  relations  of  all  phases  of  the  muscular  movement 
are  obtained  by  placing  beneath  the  lever  a  pen  atached  to  an  electro- 
magnet thrown  into  action  by  a  tuning-fork  vibrating  in  hundredths 
of  a  second.  A  marking  lever  records  simultaneously  the  moment  of 
stimulation. 

Single  Contraction. — When  a  single  electric  induction  shock  is 
applied  to  a  nerve  close  to  the  muscle,  the  latter  undergoes  a  quick 
pulsation,  speedily  returning  to  its  former  condition.  As  shown  by 


58  HUMAN    PHYSIOLOGY. 

the  muscle  curve  (see  Fig.  4),  there  is  between  the  moment  of  stimu- 
lation and  the  beginning  of  the  contraction  a  short  but  measurable 
period,  known  as  the  latent  period,  during  which  certain  chemic 
changes  are  taking  place  preparatory  to  the  exhibition  of  the  muscle 
movement.  Even  when  the  electric  stimulus  is  applied  directly  to 


FIG.  4. — MUSCLE  CURVE  PRODUCED  BY  A  SINGLE  INDUCTION  SHOCK  APPLIED  TO 

A  MUSCLE. —  (Landois.) 
a— f.  Abscissa,     a— c.  Ordinate.    a— b.  Period  of  latent  stimulation,     b— d.  Period 

of   increasing  energy,      d— e.    Period   of   decreasing   energy,      e— f.    Elastic 

after-vibrations. 

the  muscle,  a  latent  period,  though  shorter,  is  observable.  The 
duration  of  this  period  in  the  skeletal  muscles  of  the  frog  has  been 
estimated  at  o.oi  of  a  second;  but  it  has  been  shown  by  the  em- 
ployment of  more  accurate  methods  and  the  elimination  of  various 
external  influences  to  be  much  less — not  more  than  0.0033  to  0.0025 
of  a  second. 

The  contraction  follows  the  latent  period.  This  begins  slowly, 
rapidly  reaches  its  maximum,  and  ceases.  This  has  been  termed  the 
stage  of  rising  or  increasing  energy.  The  time  occupied  in  the  stage 
of  shortening  is  about  0.04  of  a  second,  though  this  will  depend  on 
the  strength  of  the  stimulus,  the  load  with  which  the  muscle  is 
weighted,  and  the  condition  of  the  muscle  irritability. 

The  relaxation  immediately  follows  the  contraction.  This  takes 
place  at  first  slowly,  after  which  the  muscle  rapidly  returns  to  its 
original  length.  This  is  the  period  of  falling  or  decreasing  energy, 
and  occupies  about  0.05  of  a  second.  The  whole  duration  of  a  muscle 
contraction  occupies,  therefore,  about  o.i  of  a  second. 

Residual  or  after-vibrations  are  frequently  seen  which  are  due  to 
changes  in  the  elasticity  of  the  muscle.  The  amplitude  of  the  con- 
traction depends  upon  the  condition  of  the  muscle,  the  load,  the 
strength  of  stimulus,  etc. 

Contraction  of  Non-striated  Muscle. — The  curve  obtained  by 
registration  of  the  contraction  of  non-striated  muscle  shows  that  it 


PHYSIOLOGY   OF    MUSCLE   TISSUE.  59 

is  similar  in  many  respects  to  that  of  the  striated  muscle,  except  that 
the  duration  of  the  former  is  considerably  longer  than  that  of  the 
latter. 

Action  of  Successive  Stimuli. — If  a  series  of  successive  stimuli 
be  applied  to  a  muscle,  the  effect  will  be  different  according  to  the 
rapidity  with  which  they  follow  one  another.  If  the  second  stimulus 
be  applied  at  the  termination  of  the  contraction  due  to  the  first 
stimulus,  a  second  contraction  follows,  similar  in  all  respects  to  the 
first.  A  third  stimulus  produces  a  third  contraction,  and  so  on  until 
the  muscle  becomes  exhausted.  If  the  second  stimulus  be  applied 
during  either  of  the  two  periods  of  the  first  contraction,  the  effects 
of  the  two  stimuli  will  be  added  together  and  the  second  contraction 
will  add  itself  to  the  first.  The  maximum  contraction  is  obtained 
when  the  second  stimulus  is  applied  ^  of  a  second  after  the  first. 

Tetanus. — When  a  series  of  stimuli  are  applied  to  a  muscle,  fol- 
lowing one  another  with  median  rapidity,  the  muscle  does  not  get 
time  to  relax  in  the  intervals  of  stimulation,  but  remains  in  a  state 
of  vibratory  contraction,  which  may  be  regarded  as  incipient  tetanus, 
or  clonus.  As  the  stimulation  increases  in  frequency,  the  vibrations 
become  invisible,  being  completely  fused  together.  There  is,  never- 
theless, during  the  tetanic  condition  a  series  of  continuous  contrac- 
tions and  relaxations  taking  place.  After  a  varying  length  of  time 
the  muscle  becomes  fatigued,  and  notwithstanding  the  stimulation, 
begins  slowly  to  elongate.  The  number  of  stimuli  necessary  a  second 
for  the  production  of  tetanus  varies  in  different  animals — e.  g.,  2  to  3 
for  muscles  of  the  tortoise;  10  for  muscles  of  the  rabbit;  15  to  20 
for  the  frog ;  70  to  80  for  birds  ;  330  to  340  for  insects. 

A  voluntary  contraction  in  man  may  be  regarded  as  a  state  of 
tetanus,  for  if  the  curve  of  a  voluntary  movement  be  examined,  it 
will  be  found  to  consist  of  intermittent  vibrations.  The  simplest 
voluntary  movement  of  a  muscle,  however  rapidly  it  may  take  place, 
last  longer  than  a  single  muscular  contraction  due  to  an  induction 
shock.  The  most  rapid  voluntary  contraction  is  the  result  of  from 
2.5  to  4  stimulations  a  second,  and  has  a  duration  of  from  0.041  to 
0.064  of  a  second.  A  continuous  voluntary  contraction  is  an  incom- 
plete tetanus.  The  number  of  stimuli  sent  to  the  muscle  is  on  the 
average,  16  to  18  for  rapid  contractions,  8  to  12  for  slow  con- 
tractions. 


60  HUMAN   PHYSIOLOGY. 

Tne  Production  of  Heat  and  Its  Relation  to  Mechanical  Work.— 
The  transformation  of  energy  which  takes  place  during  a  muscle 
contraction,  and  which  is  dependent  upon  chemic  changes  occurring 
at  that  time,  manifests  itself  as  heat  and  mechanical  work.  While 
heat  is  being  evolved  continuously  during  the  passive  condition  of 
muscles,  the  amount  of  heat  is  largely  increased  during  general 
muscle  contraction.  A  skeletal  muscle  of  a  frog — e.  g.,  the  gas- 
trocnemius, — when  removed  from  the  body,  shows,  after  tetaniza- 
tion,  an  increase  in  its  temperature  of  from  0.14°  to  0.18°  C,  and 
after  a  single  contraction  of  from  0.001°  to  0.005°  C.  While  every 
muscular  contraction  is  attended  by  an  increase  in  heat  production, 
the  amount  so  produced  will  vary  in  accordance  with  certain  condi- 
tions— e.  g.,  tension,  work  done,  fatigue,  circulation  of  blood,  etc. 

Tension. — The  greater  the  tension  of  a  muscle,  the  greater,  other 
conditions  being  equal,  is  the  amount  of  heat  evolved.  When  the 
ends  of  a  muscle  are  fastened  so  that  no  shortening  is  possible  during 
stimulation,  the  maximum  of  heat  production  is  reached.  In  the 
tetanic  state  the  great  increase  in  temperature  is  due  to  the  tension 
of  antagonistic  and  strongly  contracted  muscles.  The  evolution  of 
heat,  therefore,  bears  a  relation  to  the  resistance  against  which  the 
muscle  is  acting. 

Mechanical  Work. — If  a  muscle  contracts,  loaded  by  a  weight  just 
sufficient  to  elongate  it  to  its  original  length,  heat  is  evolved,  but 
no  mechanical  work  is  done,  all  the  energy  liberated  manifesting  itself 
as  heat.  When  the  weight  which  has  been  lifted  is  removed  from  the 
muscle  at  the  height  of  contraction,  external  work  is  done.  In  this 
case  the  amount  of  heat  liberated  is  less,  owing  to  the  work  done, 
for  some  of  the  heat  generated  is  transformed  into  mechanical  mo- 
tion. According  to  the  law  of  the  conservation  of  energy,  the 
amount  of  heat  disappearing  should  correspond  in  heat  units  to  the 
number  of  foot-pounds  produced  by  muscular  contraction. 

Muscle  Sound. — Providing  a  muscle  be  kept  in  a  state  of  tension 
during  its  contraction,  the  intermittent  variations  of  its  tension 
cause  the  muscle  to  emit  an  audible  sound.  If  the  muscle  be  tetanized 
by  induction  shocks,  the  pitch  of  the  sound  corresponds  with  the 
number  of  stimuli  a  second.  A  voluntary  contraction  is  attended  by 
a  tone  having  a  vibration  frequency  of  about  thirty-six  a  second, 
which  is,  however,  the  first  overtone  of  the  true  muscle  tone,  which 
is  caused  by  a  contraction  frequency  of  about  eighteen  a  second. 


SPECIAL   PHYSIOLOGY   OF   MUSCLES.  61 

This  low  tone  is  inaudible,   from  the  small  number  of  vibrations  a 
second. 

Muscle  Fatigue. — Prolonged  or  excessive  muscular  activity  is 
followed  by  a  diminution  in  the  power  of  producing  work  and  by  an 
increase  in  the  duration  of  the  muscular  contractions.  Fatigue  is 
accompanied  by  a  feeling  of  stiffness,  soreness,  and  lassitude,  refer- 
able to  the  muscles  themselves.  In  the  early  stages  of  muscular 
fatigue  the  contractions  increase  in  height  and  duration,  to  be  fol- 
lowed by  a  progressive  decrease  in  height,  but  an  increase  in  dura- 
tion, until  the  muscle  becomes  exhausted.  The  cause  of  the  fatigue 
is  the  production  and  accumulation  of  decomposition  products,  such 
as  phosphoric  acid  and  phosphate  of  potassium,  CO2,  etc.  A  fatigued 
muscle  is  rapidly  restored  by  the  injection  of  arterial  blood. 

Work  Done. — Muscles  are  machines  capable  of  doing  a  certain 
amount  of  work,  by  which  is  meant  the  raising  of  a  weight  against 
gravity  or  the  overcoming  of  some  resistance.  The  work  done  is  calcu- 
lated by  multiplying  the  weight  by  the  distance  through  which  it  is 
raised.  Thus,  if  a  muscle  shortens  four  millimeters  and  raises 
250  grams,  it  does  work  equal  to  1,000  milligram-meters,  or  one 
gram-meter.  If  a  muscle  contracts  without  being  weighted,  no  work 
is  done.  Equally,  when  the  muscle  is  over-weighted  so  that  it  is 
unable  to  contract,  no  work  it  done.  The  amount  of  work  a  muscle 
can  do  will  depend  upon  the  area  of  its  transverse  section,  the  length 
of  its  fibers,  and  the  amount  of  the  weight.  The  amount  of  work 
a  laborer  of  70  kilograms  weight  performs  in  eight  hours  averages 
105,605  kilogram-meters,  or  340.2  foot-tons. 


.     SPECIAL    PHYSIOLOGY    OF    MUSCLES. 

The  individual  muscles  of  the  axial  and  appendicular  portions  of 
the  body  are  named  with  reference  to  their  shape,  action,  structure, 
etc. — e.  g.,  deltoid,  flexor,  penniform,  etc.  In  different  localities  a 
group  of  muscles  having  a  common  function  is  named  in  accordance 
with  the  kind  of  motion  it  produces  or  gives  rise  to — e.  g.,  groups 
of  muscles  which  alternately  behd  or  straighten  a  joint,  or  alter- 
nately diminish  or  increase  the  angular  distance  between  two  bones, 
are  known  respectively  as  flexors  and  extensors ;  such  muscle  groups 
are  in  association  with  ginglymus  joints.  Muscles  which  turn  the 


62  HUMAN    PHYSIOLOGY. 

bone  to  which  they  are  attached  around  its  own  axis  without  pro- 
ducing any.  great  change  of  position  are  known  as  rotators,  and  are 
in  association  with  the  enarthrodial  or  ball-and-socket  joints.  Muscles 
which  impart  an  angular  movement  of  the  extremities  to  and  from 
the  median  line  of  the  body  are  termed  abductors  and  adductors. 

In  addition  to  the  actions  of  individual  groups  of  muscles  in 
causing  special  movements  in  some  regions,  several  groups  of  muscles 
are  coordinated  for  the  accomplishment  of  certain  definite  functions 
— e.  g.,  muscles  of  respiration,  mastication,  expression.  The  coordina- 
tion of  axial  and  appendicular  muscles  enables  the  individual .  to 
assume  certain  postures,  such  as  standing  and  sitting ;  to  perform 
various  acts  of  locomotion,  as  walking,  running,  swimming,  etc. 

Levers. — The    function    or    special    mode    of    action    of    individual 
muscles  can  be  understood  only  when  the  bones  with  which  they  are 
connected  are  regarded  as  levers  whose  fulcra  or  fixed  points  lie  in 
the    joints    where    the    movement    takes 
place,  and  when  the  muscles  are  consid- 
ered  as   sources   of  power   for   imparting 
"  *T  movement  to  the  levers,  with  the  object 

^£  1  of     overcoming     resistance     or      raising 

^yy  P^          weights. 

^  In  mechanics,  levers  of  three  kinds  or 

•  (3)      orders    are    recognized,    according   to   the 


relative  position   of  the  fulcrum  or  axis 
FIG.   5.— THE^THREE   ORDERS    of    motioilj    the    appHed    power,    and    the 

weight  to  be  moved.      (See   Fig.   5.) 

In  levers  of  the  first  order  the  fulcrum,  F,  lies  between  the  weight 
or  resistance,  W,  and  the  power  of  moving  force,  P.  The  distance 
P-F  is  known  as  the  power  arm,  the  distance  W-F  as  the  weight 
arm.  As  an  example  of  this  form  of  lever  in  the  human  body  may 
be  mentioned : 

1.  The   elevation   of  the   trunk   from   the   flexed  position.      The   axis 
of  movement,  the  fulcrum,  lies  in  the  hip-joint ;  the  weight,  that 
of  the  trunk,   acting   as   if  concentrated   at   its   center   of  gravity, 
lies    between    the    shoulders ;    the   power,    the    contracting   muscles 
attached   to    the   tuberosity   of   the   ischium.      The   opposite   move- 
ment is   equally  one   of  the  first  'order,   but  the   relative  positions 
of  P  and  W  are  reversed. 

2.  The  skull  in  its  movements  backward  and  forward  upon  the  atlas. 
In  levers  of  the  second  order  the  weight  lies  between  the  power  and 


SPECIAL   PHYSIOLOGY   OF   MUSCLES.  63 

the  fulcrum.     As  an  illustration  of  this  form  of  lever  may  be  men- 
tioned : 

1.  The  depression  of  the  lower  jaw,  in  which  movement  the  fulcrum 
is   the   temporomaxillary    articulation ;    the    resistance,    the    tension 
of  the  elevator  muscles ;  the  power,  the  contraction  of  the  depressor 
muscles. 

2.  The  raising   of  the  body   on   the   toes — F  being  the  toes,   W  the 
weight -of  the  body  acting  through  the  ankle,  P  the  gastrocnemius 
muscle  acting  upon  the  heel  bone. 

In  levers  of  the  third  order  the  power  is  applied  at  a  point  lying 
between  the  fulcrum  and  the  weight.  As  examples  of  this  form  of 
lever  may  be  mentioned : 

1.  The  flexion  of  the  forearm — F  being  the  elbow-joint,   P  the  con- 
tracting   biceps    and    brachialis    anticus    muscles    applied    at    their 
insertion,  W  the  weight  of  the  forearm  and  hand. 

2.  The  extension  of  the  leg  on  the  thigh. 

When  levers  are  employed  in  mechanics,  the  object  aimed  at  is  the 
overcoming  of  a  great  resistance  by  the  application  of  a  small  force 
acting  through  a  great  space,  so  as  to  obtain  a  mechanical  advantage. 
In  the  mechanism  of  the  human  body  the  reverse  generally  obtains — 
viz.,  the  overcoming  of  a  small  resistance  by  the  application  of  a 
great  force  acting  through  a  small  space.  As  a  result,  there  is  a 
gain  in  the  extent  and  rapidity  of  movement  of  the  lever.  The  power, 
however,  owing  to  its  point  of  application,  acts  at  a  great  mechanical 
disadvantage  in  many  instances,  especially  in  levers  of  the  third 
order. 

Postures. — Owing  to  its  system  of  joints,  levers,  and  muscles,  the 
human  body  can  assume  a  series  of  positions  of  equilibrium,  such  as 
standing  and  sitting,  to  which  the  name  posture  has  been  given. 
In  order  that  the  body  may  remain  in  a  state  of  stable  equilibrium 
in  any  posture,  it  is  essential  that  the  vertical  line  passing  through 
the  center  of  gravity  shall  fall  within  the  base  of  support. 

Standing  is  that  position  of  equilibrium  in  which  a  line  drawn 
through  the  center  of  gravity  falls  within  the  area  of  both  feet  placed 
on  the  ground.  This  position  is  maintained : 

1.  By  firmly  fixing  the  head  on  the  top  of  the  vertebral  column  by 
the  action  of  the  muscles  on  the  back  of  the  neck. 

2.  By    making    the    vertebral    column    rigid,    which    is    accomplished 
by    the    longissimus    dorsi    and    the    quadratus    lumborum    muscles. 


64  HUMAN   PHYSIOLOGY. 

This  having  been  accomplished,  the  center  of  gravity  falls  in  front 
of  the  tenth  dorsal  vertebra;  the  vertical  line  passing  through  this 
point  falls  behind  the  line  connecting  both  hip- joints.  In  conse- 
quence, the  trunk  is  not  balanced  on  the  hip-joints,  and  would 
fall  backward  were  it  not  prevented  by  the  contraction  of  the 
rectus  femoris  muscle  and  ligaments.  At  the  knees  and  ankles 
a  similar  balancing  of  the  parts  above  is  brought  about  by  the  action 
of  various  muscles.  When  the  entire  body  is  in  the  erect  or 
military  position,  the  arms  by  the  sides,  the  center  of  gravity 
lies  between  the  sacrum  and  the  last  lumbar  vertebra,  and  the 
vertical  line  touches  the  ground  between  the  feet  and  within  the 
base  of  support. 

Sitting  erect  is  a  condition  of  equilibrium  in  which  the"  body  is 
balanced  on  the  tubera  ischii,  when  the  trunk  and  head  together  form 
a  rigid  column.  The  vertical  line  passes  between  the  tubera. 

Locomotion  is  the  act  of  transferring  the  body,  as  a  whole,  through 
space,  and  is  accomplished  by  the  combined  action  of  its  own 
muscles.  The  acts  involved  consist  of  walking,  running,  jumping,  etc. 

Walking  is  a  complicated  act,  involving  almost  all  the  voluntary 
muscles  of  the  body,  either  for  purposes  of  progression  or  for  balanc- 
ing the  head  and  trunk,  and  may  be  denned  as  a  progression  in  a 
forward  horizontal  direction,  due  to  the-  alternate  action  of  both 
legs.  In  walking,  one  leg  becomes  for  the  time  being,  the  active  or 
supporting  leg,  carrying  the  trunk  and  head ;  the  other,  the  passive 
but  progressive  leg,  to  become  in  turn  the  active  leg  when  the  foot 
touches  the  ground.  Each  leg,  therefore,  is  alternately  in  an  active 
and  a  passive  state. 

Running  is  distinguished  from  walking  by  the  fact  that,  at  a  given 
moment,  both  feet  are  off  the  ground  and  the  body  is  raised  in  the  air. 

While  the  limits  of  a  compend  do  not  permit  of  a  description  of 
the  origin,  insertion,  and  mode  of  action  of  the  individual  muscles 
of  the  body,  it  has  been  thought  desirable  to  call  attention  to  a  few 
of  the  principal  muscles  whose  function  it  is  to  produce  special  forms 
of  movement,  as  well  as  locomotion.  (See  Fig.  6.)  The  erect  posi- 
tion is  largely  maintained  by  the  fixation  of  the  spinal  column  and 
the  balancing  of  the  head  upon  its  upper  extremity ;  the  former  is 
accompanied  by  the  erector  spincz  muscle,  named  from  its  function 
and  its  fleshy  continuations,  situated  on  each  side  of  the  vertebral 
column.  Arising  from  the  pelvis  and  lumbar  vertebrae,  this  muscle 


SPECIAL   PHYSIOLOGY   OF    MUSCLES.  65 

passes  upward,  and  is  attached  by  its  continuations  to  all  the  ver- 
tebrae. Its  action  is  to  extend  the  vertebral  column  and  to  maintain 
the  erect  position.  The  head  is  balanced  upon  the  top  of  the  ver- 
tebral column  by  the  combined  action  of  the  trapezius  and  suboccipi- 
tal  muscles  forming  the  nape  of  the  neck,  and  by  the  sterno-cleido- 
mastoid  muscle.  This  latter  muscle  arises  from  the  inner  third  of 
the  clavicle  and  upper  border  of  the  sternum.  It  is  inserted  into  the 
temporal  bone  just  behind  the  ear.  Its  action  is  to  flex  the  head 
laterally  and  to  rotate  the  face  to  the  opposite  side.  When  both 
muscles  act  simultaneously,  the  head  and  neck  are  flexed  upon  the 
thorax. 

The  temporal  and  masseter  muscles,  situated  at  the  side  of  the 
head,  arise  respectively  from  the  temporal  fossa  and  the  zygomatic 
arch,  and  are  inserted  into  the  ramus  of  the  lower  jaw.  Their  action 
is  to  close  the  mouth  and  to  assist  in  mastication.  The  occipito- 
frontalis,  the  orbicularis  palpebrarum,  and  orbicularis  oris  muscles 
are  largely  concerned  in  wrinkling  the  forehead,  closing  the  eyes  and 
mouth,  and  in  giving  various  expressions  to  the  face. 

The  deltoid  is  a  thick,  triangular  muscle  covering  the  shoulder- 
joint.  Arising  from  the  outer  third  of  the  clavicle,  the  acromial 
process,  and  the  spine  of  the  scapula,  its  fibers  converge  to  be  in- 
serted into  the  humerus  just  above  its  middle  point.  Its  action  is  to 
elevate  the  arm  through  a  right  angle.  Owing  to  its  point  of  inser- 
tion it  acts  as  a  lever  of  the  third  order,  but,  notwithstanding  the 
advantageous  points  of  insertion,  it  acts  at  a  considerable  disad- 
vantage, owing  to  the  obliquity  of  its  direction. 

The  biceps  muscle,  situated  on  the  anterior  aspect  of  the  arm,  arises 
from  the  upper  border  of  the  glenoid  fossa  and  the  coracoid  process, 
and  is  inserted  into  the  radius  just  beyond  the  elbow-joint.  Its 
action  is  to  flex  and  supinate  the  forearm  and  to  place  it  in  the  most 
favorable  position  for  striking  a  blow.  When  the  forearm  is  fixed, 
it  assists  in  flexing  the  arm,  as  in  climbing. 

The  triceps  muscle,  situated  on  the  back  of  the  arm,  arises  from 
the  scapula  and  the  posterior  surface  of  the  humerus,  and  is  inserted 
in  the  olecranon  process  of  the  ulna.  In  its  action  it  directly  an- 
tagonizes the  biceps,  namely,  extending  the  forearm.  In  so  doing  it 
acts  as  a  lever  of  the  first  order.  The  short  distance  between  the 
muscular  insertion  and  the  fulcrum  causes  it  to  act  at  a  great 
mechanical  disadvantage,  but  there  is  a  corresponding  gain  in  both 
speed  and  range  of  movement.  The  muscles  of  the  forearm  are 
6 


66 


HUMAN   PHYSIOLOGY. 


FIG.  6.— SUPERFICIAL  MUSCLES  OF  THE  BODY, 


SPECIAL   PHYSIOLOGY   OF   MUSCLES.  67 

very  numerous.  Their  action  is  to  impart  to  the  forearm  and  hand  a 
variety  of  movements,  such  as  pronation,  supination,  flexion,  ex- 
tension, rotation,  etc. 

The  pectoralis  major  and  pectoralis  minor  muscles  form  the  fleshy 
masses  of  the  breast.  Arising  from  the  inner  half  of  the  clavicle,  the 
side  of  the  sternum,  and  the  outer  surfaces  of  the  third,  fourth,  and 
fifth  ribs  anteriorly,  the  muscle-fibers  converge  to  be  inserted  into 
the  humerus  and  coracoid  process.  Their  combined  action  is  to 
adduct,  flex  and  rotate  the  arm  inward,  and  to  draw  the  scapula  down- 
ward and  forward,  movements  necessary  to  the  folding  of  the  arms 
across  the  chest. 

The  rectus  abdominis  and  the  obliquus  externus  assist  in  forming 
the  abdominal  walls. 

The  glutei  muscles  are  three  in  number,  are  arranged  in  layers,  and 
form  the  fleshy  masses  known  as  the  buttocks.  They  arise  from  the 
side  of  the  pelvis  and  are  attached  to  the  femur  in  the  neighborhood 
of  the  great  trochanter.  Their  action  is  to  extend  the  hips,  to  raise 
the  body  from  the  stooping  position,  and  to  assist  in  walking  by 
firmly  holding  the  pelvis  on  the  thigh  while  the  opposite  leg  is  ad- 
vanced in  the  forward  direction. 

The  rectus  femoris,  with  its  associates,  the  rectus  internus  and 
rectus  externus  and  the  crureus,  forms  the  fleshy  mass  on  the  an- 
terior surface  of  the  thigh.  The  former  arises  from  the  anterior 
part  of  the  ilium,  the  latter  from  the  femur.  Their  common  tendon, 
which  is  united  to  the  patella,  is  continued  as  the  ligamentum  patellae, 
which  is  attached  to  the  upper  part  of  the  tibia.  The  action  of  this 
muscular  group  is  to  extend  the  leg,  to  flex  the  thigh,  and  to  raise 
the  entire  weight  of  the  body,  as  in  changing  from  the  sitting  to  the 
erect  position. 

The  biceps  femoris  muscle,  situated  on  the  outer  and  posterior 
aspect  of  the  thigh,  arises  from  the  tuber  ischii,  and  is  inserted  into 
the  head  of  the  fibula. 

The  semimembranosus  and  the  semitendinosus  muscles,  -situated 
on  the  inner  and  posterior  aspect  of  the  thigh,  are  inserted  into  the 
head  of  the  tibia.  Their  combined  action  is  to  extend  the  hips  and 
to  flex  the  knee.  Acting  from  below,  they  assist  in  raising  the 
body  from  the  stooping  position. 

The  gastrocnemius  muscle  forms  the  enlargement  known  as  the 
calf  of  the  leg.  It  arises  by  two  heads  from  the  condyles  of  the 
femur.  Its  tendon,  the  tendo  Achillis,  is  inserted  into  the  posterior 


68  HUMAN   PHYSIOLOGY. 

surface  of  the  heel  bone.  Its  action  is  to  extend  the  foot  and  to 
raise  the  weight  of  the  body  in  walking  and  running.  On  the  front 
of  the  leg  are  numerous  muscles — e.  g.,  tibialis  anticus,  peroneus 
longus,  etc.,  the  action  of  which  is  to  flex  the  foot  and  to  antagonize 
the  gastrocnemius. 


PHYSIOLOGY    OF    NERVE   TISSUE. 

The  nerve  tissue,  which  unites  and  coordinates  the  various  organs 
and  tissues  of  the  body  and  brings  the  individual  into  relationship 
with  the  external  world,  is  arranged  anatomically  into  two  systems, 
termed  the  encephalo  or  cerebro-spinal  and  the  sympathetic. 

The  encephalo  or  cerebro-spinal  system  consists  of: 

1.  The  brain    and   spinal  cord,   contained  within  the   cavities   of   the 
cranium  and  the  spinal  column  respectively,  and 

2.  The  cranial  and  spinal  nerves. 

The  sympathetic  system  consists  of : 

1.  A   double   chain    of   ganglia   situated   on    each    side    of   the   spinal 
column   and   extending   from   the   base   of   the   skull   to   the   tip   of 
the  coccyx. 

2.  Various  collections  of  ganglia  situated  in  the  head,   face,   thorax, 
abdomen,    and   pelvis.      All   these   ganglia   are   united   by   an    elab- 
orate   system    of   intercommunicating   nerves,    many   of   which   are 
connected  with  the  cerebro-spinal  system. 

HISTOLOGY    OF    NERVE    TISSUE. 

The  Neuron. — The  nerve  tissue  has  been  resolved  by  the  investi- 
gations of  modern  histologists  into  a  single  morphologic  unit,  to  which 
the  term  neuron  has  been  applied.  The  entire  nervous  system  has 
been  shown  to  be  but  an  aggregate  of  an  infinite  number  of  neurons, 
each  of  which  is  histologically  distinct  and  independent.  Though 
having  a  common  origin,  as  shown  by  embryologic  investigations, 
they  have  acquired  a  variety  of  forms  in  different  parts  of  the 
nervous  system  in  the  course  of  development.  The  old  conception 
that  the  nervous  system  consisted  of  two  distinct  histologic  elements, 
nerve-cells  and  nerve-fibers,  which  differed  not  only  in  their  mode 
of  origin,  but  also  in  their  properties,  their  relation  to  each  other, 
and  their  functions,  has  been  entirely  disproved. 


PHYSIOLOGY   OF    NERVE   TISSUE.  69 

The  neuron,  or  neurologic  unit,  is  histologically  a  nerve-cell,  the 
surface  of  which  presents  a  greater  or  less  number  of  processes  in 
varying  degrees  of  differentiation.  As  represented  in  figure  7,  the 
neuron  may  be  said  to  consist  of:  (i)  The  nerve-cell,  neurocyte,  or 
corpus;  (2)  the  axon,  or  nerve  process;  (3)  the  end  tufts,  or  ter- 
minal branches.  Though  these  three  main  histologic  features  are 
everywhere  recognizable,  they  exhibit  a  variety  of  secondary  features 
in  different  situations  in  accordance  with  peculiarities  of  function. 
Though  the  nerve-cell  and  the  nerve-fiber  are  but  part  of  the  same 
neuron,  it  is  convenient  at  present  to  describe  them  separately. 

The  Nerve-Cell. — The  nerve-cell,  or  body  of  the  neuron,  presents 
a  variety  of  shapes  and  sizes  in  different  portions  of  the  nervous 
system.  Originally  ovoid  in  shape,  it  has  acquired,  in  course  of  de- 
velopment, peculiarities  of  form  which  are  described  as  pyramidal, 
stellate,  pear-shaped,  spindle-shaped,  etc.  The  size  of  the  cell  varies 
considerably,  the  smallest  having  a  diameter  of  not  more  than  -g-foft 
of  an  inch,  the  largest  not  more  than  ¥J^  of  an  inch.  Each  cell 
consists  of  granular,  striated  protoplasm,  containing  a  distinct  vesicu- 
lar nucleus  and  a  well-defined  nucleolus.  A  cell  membrane  has  not 
been  observed.  From  the  surface  of  the  adult  cell  portions  of  the 
protoplasm  are  projected  in  various  directions,  which  portions, 
rapidly  dividing  and  subdividing,  form  a  series  of  branches,  termed 
dendrites  or  dcndrons.  In  some  situations  the  ultimate  branches 
ol  the  dendrites  present  short  lateral  processes,  known  as  lateral 
buds,  or  gemmules,  which  impart  to  the  branches  a  feathery  appear- 
ance. This  characteristic  is  common  to  the  cells  of  the  cortex,  of 
the  cerebrum,  and  of  the  cerebellum.  The  ultimate  branches  of  the 
dendrites,  though  forming  an  intricate  feltwork,  never  anastomose 
with  one  another,  nor  unite  with  dendrites  of  adjoining  cells.  Ac- 
cording to  the  number  of  axons,  nerve-cells  are  classified  as  monax- 
onic,  diaxonic,  polyaxonic:  Most  of  the  cells  of  the  nervous  system 
of  the  higher  vertebrates  are  monaxonic.  In  the  ganglia  of  the  pos- 
terior or  dorsal  roots  of  the  spinal  and  cranial  nerves,  however,  they 
are  diaxonic.  In  this  situation  the  axons,  emerging  from  opposite 
poles  of  the  cell,  either  remain  separate  and  pursue  opposite  direc- 
tions, or  unite  to  form  a  common  stem,  which  subsequently  divides 
into  two  branches,  which  then  pursue  opposite  directions.  (See  Fig. 
7.)  The  nerve-cell  maintains  its  own  nutrition,  and  presides  over 
t.-at  of  the  dendrites  and  the  axon  as  well.  If  the  latter  be  separated 
in  any  part  of  its  course  from  the  cell,  it  speedily  degenerates  and  dies. 


70 


HUMAN   PHYSIOLOGY. 


The  axon,  or  nerve  process,  arises  from  a  cone-shaped  projection 
from  the  surface  of  the  cell,  and  is  the  first  outgrowth  from  its 
protoplasm.  At  a  short  distance  from  its  origin  it  becomes  markedly 
differentiated  from  the  dendrites  which  subsequently  develop.  It  is 
characterized  by  a  sharp,  regular  outline,  a  uniform  diameter,  and 
a  hyaline  appearance.  In  structure,  the  axon  appears  to  consist  of 
fine  fibrillse  embedded  in  a  clear,  protoplasmic  substance.  Shafer 
advocates  the  view  that  the  fibrillae  are  exceedingly  fine  tubes  filled 


Dendrites. 


Nerve-cell. 


Nerve    process 
or    axon. 


Neurilemma 


Medulla, 


Neurilemma. 


Nerve-cell. 


FIG.    7. 
A.  Efferent  neuron.      B.  Afferent  neuron. 

with  fluid.  The  axon  varies  in  length  from  a  few  millimeters  to 
100  cm.  In  the  former  instance  the  axon,  at  a  short  distance  from 
its  origin,  divides  into  a  number  of  branches,  which  form  an  intri- 
cate feltwork  in  the  neighborhood  of  the  cell.  In  the  latter  instance 
the  axon  continues  for  an  indefinite  distance  as  an  individual  struc- 


PHYSIOLOGY  OF   NERVE  TISSUE.  71 

ture.  In  its  course,  however,  especially  in  the  central  nervous  system, 
it  gives  off  a  number  of  collateral  branches,  which  possess  all  its 
histologic  features.  The  long  axons  seem  to  bring  the  body  of  the 
cell  into  direct  relation  with  peripheral  organs,  or  with  more  or  less 
remote  portions  of  the  nervous  system,  thus  constituting  association 
or  commissural  fibers. 

The  more  or  less  elongated  axon  becomes  invested,  as  a  rule,  at 
a  short  distance  from  the  cell  with  nucleated  oblong  cells,  which  sub- 
sequently become  modified  and  constitute  a  medullary  or  myelin 
sheath.'  This  is  invested  by  a  thin,  cellular  membrane — the  neuri- 
lemma.  These  three  structures  thus  constitute  what  is  known  as  a 
medullated  nerve-fiber.  In  the  central  nervous  system  the  outer 
sheath  is  frequently  absent.  In  the  sympathetic  system  the  myelin 
is  frequently  absent,  though  the  axon  is  inclosed  by  the  neurilemma, 
thus  constituting  a  non-medullated  nerve-fiber. 

The  end  tufts  or  terminal  organs  are  formed  by  the  splitting  of  the 
axon  into  a  number  of  filaments,  which  remain  independent  of  one 
another  and  are  free  from  the  medullary  investment.  The  histologic 
peculiarities  of  the  terminal  organs  vary  in  different  situations,  and 
in  many  instances  are  quite  complex  and  characteristic.  In  peripheral 
organs,  as  muscles,  glands,  blood-vessels,  skin,  mucous  membrane, 
the  tufts  are  in  direct  organic  connection  with  their  cellular  ele- 
ments. In  the  central  nervous  system  the  tufts  are  in  more  or  less 
intimate  relation  with  the  dendrites  of  adjacent  neurons. 

Nerve-fibers. — The  axons  with  their  secondary  investments  to- 
gether constitute  the  nerve-fibers,  and  according  as  they  possess  or  do 
not  possess  the  medullary  sheath,  they  may  be  divided  into  two 
groups — viz.,  medullated  and  non-medullated  fibers. 

Medullated  Nerve-fibers.— These  consist  for  the  most  part  of 
three  distinct  structures  : 

1.  An  external  investing  sheath,  tubular  in  shape,  termed  the  neuri- 
lemma. 

2.  An  intermediate  semifluid  substance — the  medulla  or  myelin. 

3.  An  internal  dark  thread — the  axis-cylinder. 

The  neurilemma  is  a  thin,  transparent,  homogeneous  membrane 
closely  adherent  to  the  medulla.  Owing  to  its  colorless  appearance,  it 
can  be  seen  only  with  difficulty  in  the  recent  condition.  When 
treated  with  various  reagents,  it  becomes  distinct.  Physically,  it  is 
quite  resistant  and  elastic.  Its  function  is  doubtless  that  of  a  pro- 
tective agent  to  the  structures  within. 


72  HUMAN    PHYSIOLOGY. 

The  medulla,  myelin,  or  white  substance  of  Schwann  completely 
fills  the  neurilemma  and  closely  invests  the  axis-cylinder.  In  the 
recent  condition  the  medulla  is  clear,  homogeneous,  semifluid,  and 
highly  refracting.  In  composition  it  is  oleaginous.  When  the  nerve 
is  treated  with  various  reagents  which  alter  its  composition,  the 
medulla  becomes  opaque  and  imparts  to  the  nerve  a  white,  glistening 
appearance.  The  function  of  the  medulla  is  quite  unknown. 

At  intervals  of  about  seventy-five  times  its  diameter  the  medullated 
nerve-fiber  undergoes  a  remarkable  diminution  in  size,  due  to  an 
interruption  of  the  medullary  substance,  so  that  the  neurilemma  lies 
directly  on  the  axis-cylinder.  These  constrictions,  or  nodes  of 
Ranvier,  taking  their  name  from  their  discoverer,  occur  at  regular 
intervals  along  the  course  of  the  nerve,  separating  it  into  a  series 
of  segments.  The  portion  between  the  nodes  is  termed  the  inter- 
nodal  segment.  It  has  been  suggested  that  in  consequence  of  the 
absence  of  the  myelin  at  these  nodes,  a  free  exchange  of  nutritive 
material  and  decomposition  products  can  take  place  between  the 
axis-cylinder  and  the  surrounding  plasma. 

The  axis-cylinder,  or  axon,  the  direct  outgrowth  of  the  nerve-cell, 
is  the  most  essential  element  of  the  nerve-fiber,  as  it  alone  is  uniformly 
continuous  throughout.  In  the  natural  condition  it  is  transparent 
and  invisible ;  but  when '  treated  with  proper  reagents,  it  presents 
itself  as  a  pale,  granular,  flattened  band,  more  or  less  solid  and  some- 
what elastic.  It  is  albuminous  in  composition.  With  high  magni- 
fication the  axis  presents  a  longitudinal  striation,  indicating  a  fibrillar 
structure.  The  fibrillse  appear  to  be  united  by  an  intervening  cement 
substance. 

Non-medullated  Nerve-fibers. — These  consist,  for  the  most  part, 
only  of  the  axis-cylinder,  though  in  some  portions  of  the  nervous 
system  a  neurilemma  is  also  present.  Though  much  less  abundant 
than  the  former  variety,  they  are  distributed  largely  throughout  the 
nervous  system,  but  are  particularly  abundant  in  the  sympathetic 
system.  Owing  to  the  absence  of  a  medulla,  they  present  a  rather 
pale  or  grayish  appearance. 

Structure  of  Nerve  Trunks. — After  their  emergence  from  the 
brain  and  spinal  cord,  the  nerve-fibers  become  bound  together,  by 
connective  tissue,  into  the  form  of  continuous  bundles,  which  connect 
the  brain  and  cord  with  all  the  remaining  structures  of  the  body.  The 
bundles  are  technically  known  as  nerve  trunks  or  nerves.  Each 


PHYSIOLOGY   OF    NERVE   TISSUE. 


73 


nerve  is  invested  by  a  thick  layer  of  lamellated  connective  tissue, 
known  as  the  epineurium.  A  transverse  section  of  a  nerve  shows 
(see  Fig.  8)  that  it  is  made  up  of  a  number  of  small  bundles  of  fibers, 
each  of  which  possesses  a  separate  investment  of  connective  tissue — 
the  perineurium.  Within  this  membrane  the  nerve-fibers  are  sup- 
ported by  a  fine  stroma — the  cndoneurium.  After  pursuing  a  longer 
or  shorter  course,  the  nerve  trunk  gives  off  branches,  which  interlace 
very  freely  with  neighboring  branches,  forming  plexuses,  the  fibers 
of  which  are  distributed  to  associated  organs  and  regions  of  the 


FIG.   8. — TRANSVERSE   SECTION   OF  A    NERVE    (MEDIAN). 
ep.    Epineurium.      pe.    Perineurium.      ed.    Endoneurium. 

body.  From  their  origin  to  their  termination,  however,  nerve- 
fibers  retain  their  individuality,  and  never  become  blended  with  ad- 
joining fibers. 

As  nerves  pass  from  their  origin  to  their  peripheral  terminations, 
they  give  off  a  number  of  branches,  each  of  which  becomes  invested 
with  a  lamellated  sheath — an  offshoot  from  that  investing  the  parent 
trunk.  This  division  of  nerve  bundles  and  sheath  continues  through- 
out all  the  branches  down  to  the  ultimate  nerve-fibers,  each  of  which 


74  HUMAN   PHYSIOLOGY. 

is  surrounded  by  a  sheath  of  its  own,  consisting  of  a  single  layer 
of  endothelial  cells.  This  delicate  transparent  membrane,  the  sheath 
of  Henle,  is  separated  from  the  nerve-fiber  by  a  considerable  space, 
in  which  is  contained  lymph  destined  for  the  nutrition  of  the  fiber. 
Near  their  ultimate  terminations  the  nerve-fibers  themselves  undergo 
division,  so  that  a  single  fiber  may  give  origin  to  a  number  of 
branches,  each  of  which  contains  a  portion  of  the  parent  axis- 
cylinder  and  myelin. 

CLASSIFICATION    OF    NERVES. 

Nerves  are  channels  of  communication  between  the  brain  and 
spinal  cord,  on  the  one  hand,  and  the  muscles,  glands,  blood-vessels, 
skin,  mucous  membrane,  viscera,  etc.,  on  the  other.  Some  of  the 
nerve-fibers  serve  for  the  transmission  of  nerve  energy  or  nerve  im- 
pulses from  the  brain  and  spinal  cord  to  certain  peripheral  organs, 
and  so  increase  or  retard  their  activities ;  others  serve  for  the 
transmission  of  nerve  energy  from  certain  peripheral  organs  to  the 
brain  and  spinal  cord,  which  gives  rise  to  sensations  or  other  modes 
of  nerve  activity.  The  former  are  termed  efferent  or  centrifugal 
nerves ;  the  latter  are  termed  afferent  or  centripetal  nerves. 

The  efferent  nerves  may  be  classified,  in  accordance  with  the  char- 
acteristic form  of  activity  to  which  they  give  rise,  into  several  groups, 
as  follows  : 

1.  Muscle    or    motor    nerves,    those    which    convey    nerve    energy    or 
nerve  impulses  to  muscles  and  give  rise  to  muscular  contraction. 

2.  Gland    or   secretory    nerves,    those    which    convey    nerve    impulses 
to  glands,  and  cause  the  formation  of  the  secretion  peculiar  to  the 
gland. 

3.  Vascular  or  vaso-motor  nerves,  those  which  convey  nerve  impulses 
to  blood-vessels,  and  cause,  either  by  stimulation  or  inhibition  of 
the  mechanism  of  their  walls,  a  contraction  (vaso-constrictors)   or 
dilatation    (vaso-dilatators)    of  the  vessel.  . 

4.  Inhibitor   nerves,    those    conveying    nerve    impulses    that    cause    a 
slowing   or   complete   cessation   of  the   rhythmic   action   of   organs. 

5.  Accelerator   nerves,    those    conveying   impulses   that   cause    an    in- 
crease in  the  rhythmic  action  of  certain  organs. 

The  afferent  nerves  may  also  be  classified,  in  accordance  with  the 
character  of  the  sensations  or  other  modes  of  nerve  activity  to 
which  they  give  rise,"  into  several  groups,  as  follows  : 


PHYSIOLOGY   OF   NERVE  TISSUE.  75 

1.  Sensorifacient   nerves,    those   conveying   nerve   impulses   that   give 
rise  in  the  brain  to  conscious  sensations.     They  may  be  subdivided 
into — 

(a)  Nerves  of  special  sense — e.  g,,  olfactory,  optic,  auditory, 
gustatory,  tactile,  thermal,  sensory,  muscle — those  which  give 
rise  to  olfactory,  optic,  auditory,  gustatory,  tactile,  thermic, 
painful,  and  muscle  sensations. 

(fr)  Nerves  of  general  sense — e.  g.,  the  visceral  afferent  nerves 
— those  which  give  rise  normally  to  vague  and  scarcely  per- 
ceptible sensations,  such  as  the  general  sensations  of  well- 
being  or  discomfort,  hunger,  thirst,  fatigue,  sex,  want  of  air, 
etc. 

2.  Reftex   nerves,   those   which   convey   nerve   impulses   to   the   nerve 
centers  and  cause  a  discharge  and  transmission  of  nerve  impulses 
outward    through    efferent    nerves    to    muscles,    glands,    or    blood- 
vessels, and  thus  influence  their  activity.     It  is  quite  probable  that 
one  and  the  same  nerve  may  subserve  both  sensational  and  reflex 
action,  owing  to  the  collateral  branches  which  are  given  off  from 
the  posterior  roots  as  they  ascend  the  posterior  column  of  the  cord. 

3.  Inhibitor  nerves,  those  which  are  capable  reflexly  of  retarding  or 
inhibiting  the  activity  of  either  nerve  centers  or  peripheral  organs. 

The  Terminal  Endings  of  Nerves. — The  efferent  nerves,  as  they 
approach  their  ultimate  terminations,  lose  both  the  neurilemma  and 
medullary  sheath.  The  axis-cylinder  then  divides  into  a  number  of 
tufts  or  branches,  which  become  directly  and  intimately  connected 
with  the  tissue  cells.  The  particular  mode  of  termination  varies  in 
different  situations.  These  terminations  are  generally  spoken  of  as 
"  end  organs." 

In  the  skeletal  muscles  the  nerve-fiber  loses  both  neurilemma  and 
myelin  sheath  at  the  point  where  it  comes  into  contact  with  the 
muscle-fiber.  After  penetrating  the  sarcolemma,  the  axis-cylinder 
breaks  up  into  small  branches  with  bulbous  extremities,  forming  the 
so-called  "  motor  plate,"  which  rests  directly  on  a  disc  of  granular 
material  containing  oval,  vesicular  nuclei.  Each  muscle-fiber  pos- 
sesses an  individual  end-plate. 

In  the  visceral  muscles  the  terminal  nerve-fibers  form  a  plexus 
around  the  muscle-fibers,  and  become  organically  connected  with 
them.  In  the  glands  the  nerve  fibers  have  been  traced  directly  to  their 
secreting  cells.  The  exact  mode  of  their  termination  and  connection 
with  the  cells  has  not  been  clearly  determined. 


76  HUMAN   PHYSIOLOGY. 

The  afferent  nerves,  as  they  approach  their  peripheral  termina- 
tions, become  connected  in  like  manner  with  end  organs,  which,  in 
some  instances,  are  extremely  complex,  such  as  those  found  in  the 
eye  (retina),  the  internal  ear,  the  nose,  and  the  tongue.  (A  con- 
sideration of  these  end  organs  will  be  found  in  the  chapters  devoted 
to  the  organs  of  which  they  form  a  part.)  The  end  organs  of  the 
skin  and  mucous  membranes  present  a  variety  of  forms,  and  may  be 
classified  as  follows  : 

1.  Free   endings    in    the    epithelium    of   the   skin,    mucous    membrane, 
and  cornea. 

2.  Tactile   cells    of    Merkel    in   the   epidermis. 

3.  Tactile  corpuscles  in  the  papilla  of  the  true  skin. 

4.  Pacinian   corpuscles    found   attached   to   the   nerves   of   the   hands 
and  feet,- to  the  intercostal  nerves,  and  to  nerves  in  other  situations. 

5.  End  bulbs  of  Krause  in  the  conjunctiva,  penis,  clitoris,  etc. 

The  end  organs  of  the  afferent  nerves  are  specialized,  highly 
irritable  structures  placed  between  the  nerve-fibers  and  the  surface 
of  the  body.  They  are  especially  adapted  for  the  reception  of  those 
external  forces  technically  known  as  stimuli,  and  for  the  liberation  of 
energy  capable  of  exciting  the  nerve-fiber  to  activity. 

Relation  of  Spinal  Nerves  to  the  Spinal  Cord. — The  nerves  in 
connection  with  the  spinal  cord  are  thirty-one  in  number,  on  each 
side  and  have  two  roots  of  origin,  an  anterior  and  a  posterior,  which 
arise  from  the  anterior  and  posterior  surfaces  of  the  cord  respectively. 
They  are  more  properly  termed  ventral  and  dorsal  roots.  The  dorsal 
roots  present,  near  their  entrance  into  the  cord,  an  enlargement 
termed  a  ganglion.  Beyond  the  spinal  canal  these  two  roots  unite 
to  form  the  ordinary  spinal  nerve.  Some  of  the  nerves  in  con- 
nection with  the  base  of  the  brain  also  present  a  ganglionic  enlarge- 
ment, and  may,  therefore,  be  regarded  physiologically  as  dorsal  nerves, 
while  others  may  be  regarded  as  ventral  nerves. 

Experimentally,  it  has  been  determined  that  the  anterior  or  ventral 
roots  contain  all  the  efferent  fibers,  the  posterior  or  dorsal  roots  all 
the  afferent  fibers.  The  proofs  in  support  of  this  view  are  as 
follows  : 

Stimulation  of  the  ventral  roots  produces  : 

1.  Convulsive  movements  of  muscles. 

2.  The  formation  of  a  secretion  in  glands. 

3.  Changes  in  the  caliber  of  blood-vessels. 


PHYSIOLOGY   OF    NERVE  TISSUE.  77 

4.  Inhibition  of  the  rhythmic  activity  of  certain  organs. 
Divisions  of  these  roots  is  followed  by : 

1.  Loss   of  muscular  movement    (paralysis   of  motion). 

2.  Cessation  of  secretion. 

3.  Cessation  of  vascular  changes. 
Stimulation   of  the   dorsal   roots   causes : 

1.  Reflex  activities. 

2.  Conscious  sensations. 

3.  Inhibition  of  the  rhythmic  activity  of  certain  organs. 
Division  of  these  roots  is  followed  by : 

1.  Loss  of  reflex  activities,  and 

2.  Loss  of  sensation  in  all  parts  to  which  they  are  distributed. 

The  ventral  roots  are,  therefore,  efferent  in  function,  transmitting 
nerve  impulses  from  the  nerve  centers  to  the  periphery.  The  dorsal 
roots  are  afferent  in  function,  transmitting  nerve  impulses  from  the 
general  periphery  to  the  nerve  centers. 

Development  and  Nutrition  of  Nerves.— The  efferent  nerve- 
fibers,  which  constitute  some  of  the  cranial  nerves  and  all  the 
ventral  roots  of  the  spinal  nerves,  have  their  origin  in  cells  located 
in  the  gray  matter  beneath  the  aqueduct  of  Sylvius,  beneath  the  floor 
of  the  fourth  ventricle  and  in  the  anterior  horns  of  the  gray  matter 
of  the  spinal  cord.  These  cells  are  the  modified  descendants  of  inde- 
pendent, oval,  pear-shaped  cells — the  neuroblasts — which  migrate  from 
the  medullary  tube.  As  they  approach  the  surface  of  the  cord  their 
axons  are  directed  toward  the  ventral  surface,  which  eventually  they 
pierce.  Emerging  from  the  cord,  the  axons  continue  to  grow,  and 
become  invested  with  the  myelin  sheath  and  neurilemma,  thus  con- 
stituting the  ventral  roots. 

The  afferent  nerve-fibers,  which  constitute  some  of  the  cranial 
nerves  and  all  the  dorsal  roots  of  the  spinal  nerves,  develop  outside 
of  the  central  nervous  system  and  only  subsequently  become  connected 
with  it.  (See  Fig.  9.)  At  the  time  of  the  closure  of  the  medullary 
tube  a  band  or  ridge  of  epithelial  tissue  develops  near  the  dorsal 
surface,  which,  becoming  segmented,  moves  outward  and  forms  the 
rudimentary  spinal  ganglia.  The  cells  in  this  situation  develop  two 
axons,  one  from  each  end  of 'the  cell,  which  pass  in  opposite  direc- 
tions, one  toward  the  spinal  cord,  the  other  toward  the  periphery. 
In  the  adult  condition  the  two  axons  shift  their  position,  unite,  and 
form  a  T-shaped  process,  after  which  a  division  into  two  branches 


78  HUMAN    PHYSIOLOGY. 

again  takes  place.     In  the  ganglia  of  all  the  sensoricranial  and  sensori- 
spinal  nerves  the  cells  have  this  histologic  peculiarity. 


Posterior 
Jbot 


FIG  9. — DIAGRAM  SHOWING  THE  MODE  OF  ORIGIN  OF  THE  VENTRAL  AND 
DORSAL  ROOTS. 

Nerve  Degeneration. — If  any  one  of  the  cranial  or  spinal  nerves 
be  divided  in  any  portion  of  its  course,  the  part  in  connection  with 
the  periphery  in  a  short  time  exhibits  certain  structural  changes,  to 
which  the  term  degeneration  is  applied.  The  portion  in  connection 
with  the  brain  or  cord  retains  its  normal  condition.  The  degenera- 
tive process  begins  simultaneously  throughout  the  entire  course  of 
the  nerve,  and  consists  in  a  disintegration  and  reduction  of  the 
medulla  and  axis-cylinder  into  nuclei,  drops  of  myelin,  and  fat,  which 
in  time  disappear  through  absorption,  leaving  the  neurilemma  intact. 
Coincident  with  these  structural  changes  there  is  a  progressive  altera- 
tion and  diminution  in  the  excitability  of  the  nerve.  Inasmuch  as 
the  central  portion  of  the  nerve,  which  retains  its  connection  with 
the  nerve-cell,  remains  histologically  normal,  it  has  been  assumed 
that  the  nerve-cells  exert  over  the  entire  course  of  the  nerve-fibers 
a  nutritive  or  a  trophic  influence.  This  idea  has  been  greatly  strength- 
ened since  the  discovery  that  the  axis-cylinder,  or  the  axon,  has  its 
origin  in  and  is  a  direct  outgrowth  of  the  cell.  When  separated  from 


PHYSIOLOGY   OF    NERVE  TISSUE.  79 

the  parent  cell,  the  fiber  appears  to  be  incapable  of  itself  of  main- 
taining its  nutrition. 

The  relation  of  the  nerve-cells  to  the  nerve-fibers,  in  reference  to 
their  nutrition,  is  demonstrated  by  the  results  which  follow  section 
of  the  ventral  and  dorsal  roots  of  the  spinal  nerves.  If  the  anterior 
root  alone  be  divided,  the  degenerative  process  is  confined  to  the 
peripheral  portion,  the  central  portion  remaining  normal.  If  the 
posterior  root  be  divided  on  the  peripheral  side  of  the  ganglion,  de- 
generation takes  place  only  in  the  peripheral  portion  of  the  nerve. 
If  the  root  be  divided  between  the  ganglion  and  the  cord,  degenera- 
tion takes  place  only  in  the  central  portion  of  the  root.  From  these 
facts  it  is  evident  that  the  trophic  centers  for  the  ventral  and  dorsal 
roots  lie  in  the  spinal  cord  and  spinal  nerve  ganglia,  respectively,  or, 
in  other  words,  in  the  cells  of  which  they  are  an  integral  part.  The 
structural  changes  which  nerves  undergo  after  separation  from  their 
centers  are  degenerative  in  character,  and  the  process  is  usually 
spoken  of,  after  its  discoverer,  as  the  Wallerian  degeneration. 

When  the  degeneration  of  the  efferent  nerves  is  completed,  the 
structures  to  which  they  are  distributed,  especially  the  muscles,  un- 
dergo an  atrophic  or  fatty  degeneration,  with  a  change  or  loss  of  their 
irritability.  This  is,  apparently,  not  to  be  attributed  merely  to  in- 
activity, but  rather  to  a  loss  of  nerve  influences,  inasmuch  as  inac- 
tivity merely  leads  to  atrophy  and  not  to  degeneration. 

Reactions  of  Degeneration. — In  consequence  of  the  degeneration 
and  changes  in  irritability  which  occur  in  nerves  when  separated 
from  their  centers  and  in  muscles  when  separated  from  their 
related  nerves,  either  experimentally  or  as  the  result  of  disease,  the 
response  of  these  structures  to  the  induced  and  the  make-and-break 
of  the  constant  currents  differs  from  that  observed  in  the  physiologic 
condition.  The  facts  observed  under  the  application  of  these  two 
forms  of  electricity  are  of  the  greatest  importance  in.  the  diagnosis 
and  therapeutics  of  the  precedent  lesions.  The  principal  difference 
of  behavior  is  observed  in  the  muscles,  which  exhibit  a  diminished 
or  abolished  excitability  to  the  induced  current,  while  at  the  same 
time  manifesting  an  increased  excitability  to  the  constant  current ; 
so  much  so  is  this  the  case  that  a  closing  contraction  is  just  as  likely 
to  occur  at  the  positive  as  at  the  negative  pole.  This  peculiarity 
of  the  muscle  response  is  termed  the  reaction  of  degeneration.  The 
synchronous  diminished  excitability  of  the  nerves  is  the  same  for 


80  HUMAN    PHYSIOLOGY. 

either  current.  The  term  "  partial  reaction  of  degeneration "  is 
used  when  there  is  a  normal  reaction  of  the  nerves,  with  the  degen- 
erative reaction  of  the  muscles.  This  condition  is  observed  in  pro- 
gressive muscular  atrophy. 

Reflex  Action. — Inasmuch  as  many  of  the  muscle  movements  of 
the  body,  as  well  as  the  formation  and  discharge  of  secretions  from 
glands,  variations  in  the  caliber  of  blood-vessels,  inhibition  and  ac- 
celeration in  the  activity  of  various  organs,  are  the  result  of  stimula- 
tions of  the  terminal  organs  of  afferent  nerves,  they  are  termed,  for 
convenience,  reflex  actions,  and,  as  they  take  place  independently  of 
the  brain  or  of  volitional  impulses,  they  are  also  termed  involuntary 
actions.  As  many  of  the  processes  to  be  described  in  succeeding 
chapters  are  of  this  character,  requiring  for  their  performance  the 
cooperation  of  several  organs  and  tissues  associated  through  the 
intermediation  of  the  nervous  system,  it  seems  advisable  to  consider 
briefly,  in  this  connection,  the  parts  involved  in  a  reflex  action,  as 
well  as  their  mode  of  action.  As  shown  in  figure  10,  the  necessary 
structures  are  as  follows  : 

i.  A  sentient  surface,  skin,  mucous  membrane,  sense  organ,  etc. 
2    An  afferent  nerve. 

3.  An  emissive  cell,  from  which  arises 

4.  An  efferent  nerve,  distributed  to  a  responsive  organ,  as 

5.  Muscle,  gland,  blood-vessel,  etc. 

Such  a  combination  of  structures  constitutes  a  reflex  mechanism  or 
arc  the  nerve  portion  of  which  is  composed  of  but  two  neurons — an 
afferent  and  an  efferent.  An  arc  of  this  simplicity  would  of  necessity 
subserve  but  a  simple  movement.  The  majority  of  reflex  activities, 
however,  are  extremely  complex,  and  involve  the  cooperation  and 
coordination  of  a  number  of  structures  frequently  situated  at  dis- 
tances more  or  less  remote  from  one  another.  This  implies  that  a 
number  of  neurons  are  associated  in  function.  The  afferent  neurons 
are  brought  into  relation  with  the  dendrites  of  the  efferent  neurons 
by  the  end  tufts  of  the  collateral  branches,  which  may  extend  for 
some  distance  up  and  down  the  cord  before  passing  into  the  various 
segments. 

For  the  excitation  of  a  reflex  action  it  is  essential  that  the  stimulus 
applied  to  the  sentient  surface  be  of  an  intensity  sufficient  to  de- 
velop in  the  terminals  of  the  afferent  nerve  a  series  of  nerve  impulses, 
which,  raveling  inward,  will  be  distributed  to  and  received  by  the 


PHYSIOLOGY   OF    NERVE   TISSUE. 


81 


dendrites  of  the  emissive  or  motor  cell.  With  the  reception  of  these 
impulses  there  is  apparently  a  disturbance  of  the  equilibrium  of  its 
molecules,  a  liberation  of  energy,  and,  in  consequence,  a  transmission 
outward  of  impulses  through  the  efferent  nerve  to  muscle,  gland,  or 
blood-vessel,  separately  or  collectively,  with  the  production  of 
muscular  contraction,  glandular  secretion,  vascular  dilatation  or  con- 
traction, etc.  The  reflex  actions  take  place,  for  the  most  part,  through 


M 


FIG    10. — DIAGRAM    ILLUSTRATING    REFLEX   ACTION. — (Kirke.) 

S.  Sentient  surface  from  which  proceeds  the  afferent  nerve.  M.  C.  Motor  or 
emissive  cell  giving  origin  to  efferent  nerve  which  terminates  in  M.  M. 
Motor  organ.  G.  Ganglion  cell  on  afferent  nerve. 

the  spinal  cord  and  medulla  oblongata,  which,  in  virtue  of  their 
contained  centers,  coordinate  the  various  organs  and  tissues  con- 
cerned in  the  performance  of  the  organic  functions.  The  movements 
of  mastication  ;  the  secretion  of  saliva ;  the  muscular,  glandular,  and 
vascular  phenomena  of  gastric  and  intestinal  digestion ;  the  vascular 
and  respiratory  movements  ;  the  mechanism  of  micturition,  etc.,  are 
illustrations  of  reflex  activity. 


82  HUMAN   PHYSIOLOGY. 

PHYSIOLOGIC    PROPERTIES    OF    NERVES. 

Nerve  Irritability  or  Excitability  and  Conductivity. — These  terms 
are  employed  to  express  that  condition  of  a  nerve  which  enables  it 
to  develop  and  to  conduct  nerve  impulses  from  the  center  to  the 
periphery,  from  the  periphery  to  the  center,  in  response  to  the  action 
of  stimuli.  A  nerve  is  said  to  be  excitable  or  irritable  as  long  as  it 
possesses  these  capabilities  or  properties.  For  the  manifestation  of 
these  properties  the  nerve  must  retain  a  state  of  physical  and  chemic 
integrity ;  it  must  undergo  no  change  in  structure  or  chemic  compo- 
sition. The  irritability  of  an  efferent  nerve  is  demonstrated  by 
the  contraction  of  a  muscle,  by  the  secretion  of  a  gland,  or  by  a 
change  in  the  caliber  of  a  blood-vessel,  whenever  a  corresponding 
nerve  is  stimulated.  The  irritability  of  an  efferent  nerve  is  demon- 
strated by  the  production  of  a  sensation  or  a  reflex  action  whenever 
it  is  stimulated.  The  irritability  of  nerves  continues  for  a  certain 
period  of  time  after  separation  from  the  nerve  centers  and  even 
after  the  death  of  the  animal,  varying  in  different  classes  of  animals. 
In  the  warm-blooded  animals,  in  which  the  nutritive  changes  take 
place  with  great  rapidity,  the  irritability  soon  disappears — a  result 
due  to  disintegrative  changes  in  the  nerve,  caused  by  the  withdrawal 
of  the  blood-supply.  In  cold-blooded  animals,  on  the  contrary,  in 
which  the  nutritive  changes  take  place  relatively  slowly,  the  irrita- 
bility lasts,  under  favorable  conditions,  for  a  considerable  time.  Other 
tissues  besides  nerves  possess  irritability,  that  is,  the  property  of 
responding  to  the  action  of  stimuli — e.  g.,  glands  and  muscles,  which 
respond  by  the  production  of  a  secretion  or  a  contraction. 

Independence  of  Tissue  Irritability. — The  irritability  of  nerves  is 
distinct  and  independent  of  the  irritability  of  muscles  and  glands,  as 
shown  by  the  fact  that  it  persists  in  each  a  variable  length  of  time 
after  their  histologic  connections  have  been  impaired  or  destroyed 
by  the  introduction  of  various  chemic  agents  into  the  circulation. 
Curara,  for  example,  induces  a  state  of  complete  paralysis  by  modify- 
ing or  depressing  the  conductivity  of  the  end  organs  of  the  nerves 
just  where  they  come  in  contact  with  the  muscles  without  impairing 
the  irritability  of  either  nerve  trunks  or  muscles.  Atropin  induces 
complete  suspension  of  glandular  activity  by  impairing  the  terminal 
organs  of  the  secretor  nerves  just  where  they  come  into  relation 
with  the  gland-cells,  without  destroying  the  irritability  of  either 
gland  or  nerve. 


PHYSIOLOGY   OF   NERVE  TISSUE.  83 

Stimuli  of  Nerves. — Nerves  do  not  possess  the  power  of  spon- 
taneously generating  and  propagating  nerve  impulses ;  they  can  be 
aroused  to  activity  only  by  the  action  of  an  extraneural  stimulus.  In 
the  living  condition  the  stimuli  capable  of  throwing  the  nerve  into 
an  active  condition  act  for  the  most  part  on  either  the  central  or 
peripheral  end  of  the  nerve.  In  the  case  of  motor  nerves  the  stimulus 
to  the  excitation,  originating  in  some  molecular  disturbance  in  the 
nerve-cells,  acts  upon  the  nerve-fibers  in  connection  with  them.  In 
the  case  of  sensor  or  afferent  nerves  the  stimuli  act  upon  the  peculiar 
end  organs  with  which  the  sensor  nerves  are  in  connection,  which 
in  turn  excite  the  nerve-fibers.  Experimentally,  it  can  be  demonstated 
that  nerves  can  be  excited  by  a  sufficiently  powerful  stimulus  applied 
in  any  part  of  their  extent. 

Nerves  respond  to  stimulation  according  to  their  habitual  func- 
tion ;  thus,  stimulation  of  a  sensor  nerve,  if  sufficiently  strong, 
results  in  the  sensation  of  pain  ;  of  the  optic  nerve,  in  the  sensation 
of  light ;  of  a  motor  nerve,  in  contraction  of  the  muscle  to  which  it 
is  distributed;  of  a  secretor  nerve,  in  the  activity  of  the  related 
gland,  etc.  It  is,  therefore,  evident  that  peculiarity  of  nerve  func- 
tion depends  neither  upon  any  special  construction  or  activity  of 
the  nerve  itself,  nor  upon  the  nature  of  the.  stimulus,  but  entirely 
upon  the  peculiarities  of  its  central  and  peripheral  end  organs. 

Nerve  stimuli  may  be  divided  into — 

1.  General   stimuli,    comprising   those   agents    which    are    capable    of 
exciting  a  nerve  in  any  part  of  its  course. 

2.  Special  stimuli,   comprising  those   agents   which   act   upon   nerves 
only  through  the  intermediation  of  the  end  organs. 

General  stimuli: 

1.  Mechanical:    as  from  a  blow,  pressure,  tension,  puncture,  etc. 

2.  Thermal :    heating  a  nerve  at  first  increases  and  then  decreases  its 
excitability. 

3.  Chemic :     sensor    nerves    respond    somewhat    less    promptly    than 
motor  nerves  to  this  form  of  irritation. 

4.  Electric :    either  the  constant  or  interrupted  current. 

5.  The  normal  physiologic  stimulus  : 

(a)   Centrifugal  or  efferent,  if  proceeding  from  the  center  toward 

the  periphery. 
(&)    Centripetal  or  afferent,  if  in  the  reverse  direction. 


84 


HUMAN   PHYSIOLOGY. 


Special  stimuli: 

1.  Light   or  ethereal   vibrations   acting  upon   the   end   organs    of   the 
optic  nerve  in  the  retina. 

2.  Sound  or  atmospheric  undulations  acting  upon  the  end  organs  of 
the  auditory  nerye. 

3.  Heat   or  vibrations   of  the  air  upon   the  end   organs  in  the   skin. 

4.  Chemic  agencies  acting  upon  the  end  organs  of  the  olfactory  and 
gustatory  nerves. 

Nature  of  the  Nerve  Impulse. — As  to  the  nature  of  the  nerve  im- 
pulse generated  by  any  of  the  foregoing  stimuli  either  general  or 
special,  but  little  is  known.  It  has  been  supposed  to  partake  of  the 
nature  of  a  molecular  disturbance,  a  combination  of  physical  and 
chemical  processes  attended  by  the  liberation  of  energy,  which  propa- 
gates itself  from  molecule  to  molecule.  Judging  from  the  deflections 
of  the  galvanometer  needle  it  is  probable  that  when  the  nerve  im- 
pulse makes  its  appearance  at  any  given  point  it  is  at  first  feeble 
but  soon  reaches  a  maximum  development  after  which  it  speedily 
declines  and  disappears.  It  may,  therefore,  be  graphically  repre- 
sented as  a  wave-like  movement  with  a  definite  length  and  time  dura- 
tion. Under  strictly  physiological  conditions  the  nerve  impulse 
passes  in  one  direction  only ;  in  efferent  nerves  from  the  center  to 
the  periphery,  in  afferent  nerves  from  the  periphery  to  the  center. 
Experimentally,  however,  it  can  be  demonstrated  that  when  a  nerve 
impulse  is  aroused  in  the  course  of  a  nerve  by  an  adequate  stimulus 
it  travels  equally  well  in  both  directions  from  the  point  of  stimula- 
tion. When  once  started  the  impulse  is  confined  to  the  single  fiber 
and  does  not  diffuse  itself  to  fibers  adjacent  to  it  in  the  same  nerve 
trunk. 

Rapidity  of  Transmission  of  Nerve  Force. — The  passage  of  a 
nervous  impulse,  either  from  the  brain  to  the  periphery  or  in  the 
reverse  direction,  requires  an  appreciable  period  of  time.  The 
velocity  with  which  the  impulse  travels  in  human  sensor  nerves 
has  been  estimated  at  about  190  feet  a  second,  and  for  motor  nerves 
at  from  100  to  200  feet  a  second.  The  rate  of  movement  is,  however, 
somewhat  modified  by  temperature,  cold  lessening  and  heat  increas- 
ing the  rapidity ;  it  is  also  modified  by  electric  conditions,  by  the 
action  of  drugs,  the  strength  of  the  stimulus,  etc.  The  rate  of  trans- 
mission through  the  spinal  cord  is  considerably  slower  than  in  nerves, 
the  average  velocity  for  voluntary  motor  impulses  being  only  33  feet 


PHYSIOLOGY   OF   NERVE  TISSUE.  85 

a  second,  for  sensitive  impressions  40  feet,  and  for  tactile  im- 
pressions 140  feet  a  second. 

Electric  Currents  in  Muscles  and  Nerves. — If  a  muscle  or  nerve 
be  divided  and  non-polarizable  electrodes  be  placed  upon  the  natural 
longitudinal  surface  at  the  equator,  and  upon  the  transverse  section, 
electric  currents  are  observed  with  the  aid  of  a  delicate  galvanometer. 
The  direction  of  the  current  is  always  from  the  positive  equatorial 
surface  to  the  negative  transverse  surface.  The  strength  of  the  cur- 
rent increases  or  diminishes  according  as  the  positive  electrode  is 
moved  toward  or  from  the  equator.  When  the  electrodes  are  placed 
on  the  two  transverse  ends  of  a  nerve,  an  axial  current  will  be 
observed  the  direction  of  which  is  opposite  to  that  of  the  normal 
impulse  of  the  nerve. 

The  electromotive  force  of  the  strongest  nerve-current  has  been 
estimated  to  be  equal  to  the  0.026  of  a  Daniell  battery ;  the  force 
of  the  current  of  the  frog  muscle,  about  0.05  to  0.08  of  a  Daniell. 

Negative  Variation  of  Currents  in  Muscles  and  Nerves. — If  a 

muscle  or  nerve  be  thrown  into  a  condition  of  tetanus,  it  will  be 
observed  that  the  currents  undergo  a  diminution  of  negative  variation, 
a  change  which  passes  along  the  nerve  in  the  form  of  a  wave  and 
with  a  velocity  equal  to  the  rate  of  transmission  of  the  nerve  im- 
pulse. The  wave-length  of  a  single  negative  variation  has  been 
estimated  to  be  eighteen  millimeters,  the  period  of  its  duration  being 
from  0.0005  to  0.0008  of  a  second. 

It  is  asserted  by  Hermann  that  perfectly  fresh,  uninjured  muscles 
and  nerves  are  devoid  of  currents,  and  that  the  currents  observed 
are  the  result  of  molecular  death  at  the  point  of  section,  this  point 
becoming  negative  to  the  equatorial  point.  He  applies  the  term 
"  action  currents  "  to  the  currents  obtained  when  a  muscle  is  thrown 
into  a  state  of  activity. 

Electrotonus. — The  passage  of  a  direct  galvanic  current  through  a 
portion  of  a  nerve  excites  in  the  parts  beyond  the  electrodes  a  con- 
dition of  electric  tension,  or  electrotonus,  during  which  the  excita- 
bility of  the  nerve  is  decreased  near  the  anode  or  positive  pole,  and 
increased  near  the  cathode  or  negative  pole  ;  the  increase  of  excita- 
bility in  the  catelectrotonic  area — that  nearest  the  muscle — being 
manifested  by  a  more  marked  contraction  of  the  muscle  than  the 
normal  when  the  nerve  is  irritated  in  this  region.  The  passage  of 
an  inverse  galvanic  current  excites  the  same  condition  of  electrotonus ; 


00  HUMAN   PHYSIOLOGY. 

the  diminution  of  excitability  near  the  anode,  the  anelectrotonic. — 
that  now  nearest  the  muscle, — being  manifested  by  a  less  marked 
contraction  than  the  normal  when  the  nerve  is  stimulated  in  this 
region.  Similar  conditions  exist  within  the  electrodes.  Between  the 
electrodes  is  a  neutral  point,  where  the  catelectrotonic  area  merges 
into  the  anelectrotonic  area.  If  the  current  be  a  strong  one,  the 
neutral  point  approaches  the  cathode ;  if  weak,  it  approaches  the 
anode. 

When  a  nerve  impulse  passes  along  a  nerve,  the  only  appreciable 
effect  is  a  change  in  its  electric  condition,  there  being  no  change  in 
its  temperature,  chemic  composition,  or  physical  condition.  The 
natural  nerve-currents,  which  are  always  present  in  a  living  nerve  as 
a  result  of  its  nutritive  activity,  in  great  part  disappear  during  the 
passage  of  an  impulse,  undergoing  a  negative  variation. 

Law  of  Contraction. — If  a  feeble  galvanic  current  be  applied  to 
a  recent  and  excitable  nerve,  contraction  is  produced  in  the  muscles 
only  upon  the  making  of  the  circuit  with  both  the  direct  and  inverse 
currents. 

If  the  current  be  moderate  in  intensity,  the  contraction  is  produced 
in  the  muscle,  both  upon  the  making  and  breaking  of  the  circuit,  with 
both  the  direct  and  inverse  currents. 

If  the  current  be  intense,  contraction  is  produced  only  when  the 
circuit  is  made  with  the  direct  current,  and  only  when  it  is  broken 
with  the  inverse  current. 


FOODS    AND    DIETETICS. 

During  the  functional  activity  of  every  organ  and  tissue  of  the 
body  the  living  material  of  which  it  is  composed — the  protoplasm — 
undergoes  more  or  less  disintegration.  Through  a  series  of  descend- 
ing chemic  stages  it  is  reduced  to  a  number  of  simpler  compounds, 
which  are  of  no  further  value  to  the  body,  and  which  are  in  conse- 
quence eliminated  by  the  various  eliminating  or  excretory  organs — 
the  lungs,  kidneys,  skin,  liver.  Among  these  compounds  the  more 
important  are  carbon  dioxid,  urea,  and  uric  acid.  Many  other  com- 
pounds, inorganic  as  well  as  organic,  are  also  eliminated  in  the 
water  discharged  from  the  body,  in  which  they  are  held  in  solution. 


FOODS   AND  DIETETICS.  Ol 

Coincident  with  this  disintegration  of  the  tissues  there  is  an  evolu- 
tion or  disengagement  of  energy,  particularly  in  the  form  of  heat. 

In  order  that  the  tissues  may  regain  their  normal  composition  and 
thus  be  enabled  to  continue  in  the  performance  of  their  functions, 
they  must  be  supplied  with  the  same  nutritive  materials  of  which 
their  protoplasm  originally  consisted — viz.,  water,  inorganic  salts, 
proteids,  sugar,  fat.  These  materials  are  furnished  by  the  blood 
during  its  passage  through  the  capillary  blood-vessels.  The  blood 
is  a  reservoir  of  nutritive  material  in  a  condition  to  be  absorbed, 
organized,  and  transformed  into  new  living  tissue. 

Inasmuch  as  the  loss  of  material  from  the  body  daily,  which  is 
very  great,  is  compensated  for  under  other  forms  by  the  blood,  it  is 
evident  that  this  fluid  would  rapidly  diminish  in  volume  were  it  not 
restored  by  the  introduction  of  new  and  corresponding  materials. 
As  soon  as  the  blood  volume  falls  to  a  certain  point,  the  sensations 
of  hunger  and  thirst  arise,  which  in  a  short  time  lead  to  the  necessity 
of  taking  food. 

In  addition  to  the  direct  appropriation  of  food  by  the  tissues  it  is 
highly  probable  that  an  indefinite  amount  undergoes  oxidation  and 
disintegration  without  ever  becoming  an  integral  part  of  the  tissues, 
and  thus  directly  contributes  to  the  production  of  heat. 

Inanition  or  Starvation. — If  these  nutritive  principles  be  not  sup- 
plied in  sufficient  quantity,  or  if  they  are  withheld  entirely,  a  condi- 
tion of  physiologic  decay  is  established,  to  which  the  term  inanition 
or  starvation  is  applied.  The  phenomena  which  characterize  this 
pathologic  process  are  as  follows — viz.,  hunger,  intense  thirst,  gastric 
and  intestinal  uneasiness  and  pain,  muscle  weakness  and  emaciation, 
a  diminution  in  the  quantity  of  carbon  dioxid  exhaled,  a  lessening 
in  the  amount  of  urine  and  its  constituents  excreted,  a  diminution  in 
the  volume  of  the  blood,  an  exhalation -of  a  fetid  odor  from  the  body, 
vertigo,  stupor,  delirium,  and  at  times  convulsions,  a  fall  of  bodily 
temperature,  and,  finally,  death  from  exhaustion. 

During  starvation  the  loss  of  different  tissues,  before  death  occurs, 
averages  T4Q,  or  40  per  cent.,  of  their  weight. 

Those  tissues  which  lose  more  than  40  per  cent,  are:  Fat,  93.3; 
blood,  75;  spleen,  71.4;  pancreas,  64.1;  liver,  52;  heart,  44.8;  in- 
testines, 42.4 ;  muscle,  42.3.  Those  which  lose  less  than  40  per  cent. 
are :  The  muscular  coat  of  the  stomach,  39.7  ;  pharynx  and  esophagus, 
34.2;  skin,  33.3;  kidneys,  31.9;  respiratory  apparatus,  22.2;  bones, 
16.7;  eyes,  10;  nervous  system,  1.9. 


88 


HUMAN   PHYSIOLOGY. 


The  fat  entirely  disappears,  with  the  exception  of  a  small  quantity 
which  remains  in  the  posterior  portion  of  the  orbits  and  around 
the  kidneys.  The  blood  diminishes  in  volume  and  loses  its  nutritive 
properties.  The  muscles  undergo  a  marked  diminution  in  volume  and 
become  soft  and  flabby.  The  nervous  system  is  last  to  suffer,  not 
more  than  two  per  cent.,  disappearing  before  death  occurs. 

The  appearances  presented  by  the  body  after  death  from  starvation 
are  those  of  anemia  and  great  emaciation ;  almost  total  absence  of 
fat ;  bloodlessness ;  a  diminution  in  the  volume  of  the  organs ;  an 
empty  condition  of  the  stomach  and  bowels,  the  coats  of  which  are 
thin  and  transparent.  There  is  a  marked  disposition  of  the  body  to 
undergo  decomposition,  giving  rise  to  a  very  fetid  odor. 

The  duration  of  life  after  a  complete  deprivation  of  food  varies 
from  eight  to  thirteen  days,  though  life  can  be  maintained  much 
longer  if  a  quantity  of  water  be  obtained.  The  water  is  more  essen- 
tial under  these  circumstances  than  the  solid  matters,  which  can  be 
supplied  by  the  organism  itself. 

The  different  alimentary  or  nutritive  principles  which  are  appro- 
priated by  the  tissues,  and  which  are  contained  within  the  various 
articles  of  food,  belong  to  both  the  organic  and  inorganic  groups  and 
chemic  compounds,  and  may  be  classified  according  to  their  composi- 
tion as  follows : 

CLASSIFICATION    OF   ALIMENTARY    PRINCIPLES. 

1.  Proteid  Group.— Nitrogenized,  C,  0,  H,  N,  S,  P. 

Principle.  Where  Found. 

Myosin Flesh  of  animals. 

Vitellin,  albumin Yolk  of  egg,  white  of  egg. 

Fibrin,  globulin Blood  contained  in  meat. 

Casein Milk,    cheese. 

Gluten Grain  of  wheat  .and  other  cereals. 

Vegetable  albumin Soft,  growing  vegetables. 

Legumin Peas,  beans,  lentils,  etc. 

Gelatin Bones. 

2.  Oleaginous  Group. — C,  0,  H. 

Animal  fats  and  oils "1   Found  in  the  adipose  tissue  of  ani- 

Stearin,  olein L      mals,  seeds,  grains,  nuts,  fruits, 

Palmitin,  fatty  acids J        and  other  vegetable  tissues. 

3.  Carbohydrate  Group. — C,  0,  H. 


FOODS    AND   DIETETICS.  89 

Saccharose,  or  cane-sugar    .     .     .       Sugar-cane. 

Dextrose,  or  glucose ,     _     . 

V   Fruits. 

Levulose,  or  fruit-sugar  .     .     .     .  j 

Lactose,  or  milk-sugar    ....       Milk. 

Maltose Malt,   malt  foods. 

Starch Cereals,  tuberous  roots,  and  legu- 
minous  plants. 
Glycogen Liver,  muscles. 

4.  Inorganic  Group. — Water ;  sodium  and  potassium  chlorids  ;  sodium 
calcium,  magnesium,  and  potassium  phosphates ;  calcium  carbonate  ; 
and  iron. 

5.  Vegetable  Acid   Group. — Malic,   citric,  tartaric,   and  other   acids, 
found  principally  in  fruits. 

6.  Accessory  Foods. — Tea,  coffee,  alcohol,  cocoa,  etc. 

The  proteid  principles  of  the  food,  after  undergoing  digestion  and 
conversion  into  peptones,  are  absorbed  and  transformed  into  the 
form  of  proteids  characteristic  of  the  blood  plasma  and  the  lymph. 
Of  the  proteids  thus  brought  into  relation  with  the  living  protoplasm, 
a  small  percentage  only  is  utilized  in  the  repair  of  its  substance. 
This  is  known  as  tissue  proteid.  A  large  percentage  circulating 
among  and  permeating  the  tissues  is  acted  upon  by  them  directly, 
and  reduced  to  simpler  compounds  without  ever  becoming  a  part 
of  the  tissue  itself.  This  is  known  as  circulating  proteid.  In  the 
process  of  tissue  metabolism  all  the  proteids  suffer  disintegration, 
and  give  rise  to  the  production  of  some  carbon-holding  compound, 
probably  fat,  and  some  nitrogen-holding  compounds  which  eventually 
produce  urea.  The  intermediate  stages  are  possibly  represented  by 
glycin,  creatin,  uric  acid,  etc.  An  excess  of  proteids  in  the  food 
is  followed  by  their  decomposition,  by  the  pancreatic  juice,  into 
leucin  and  tyrosin,  which,  by  the  agency  of  the  liver,  are  converted 
into  urea.  The  disintegration  of  the  proteids  is  attended  by  the  dis- 
engagement of  heat :  they  thus  contribute  to  the  energy  of  the  body. 

The  oleaginous  principles,  after  digestion,  are  absorbed  into  the 
blood,  from  which  they  rapidly  disappear.  It  is  probable  that  a  por- 
tion of  the  fat  enters  directly  into  the  composition  of  living  proto- 
plasm, out  of  which  it  again  emerges  at  some  subsequent  stage  in  the 
form  of  small  drops  which  make  their  appearance  in  the  protoplasmic 
cells  of  the  connective  areolar  tissue,  thus  giving  rise  to  the  adipose 
tissue.  Another  portion  probably  undergoes  direct  oxidation. 


90  HUMAN   PHYSIOLOGY. 

The  carbohydrate  principles,  after  digestion,  are  absorbed  as  dex- 
trose and  temporarily  stored  up  in  the  liver  as  glycogen.  The  inter- 
mediate stages  which  sugar  passes  through  and  the  combinations  into 
which  it  enters  between  its  absorption  and  its  elimination  are  but 
imperfectly  understood.  That  it  contributes  to  the  accumulation  of 
fat  is  probable,  though  it  is  doubtful  if  it  is  ever  converted  into  fat. 
A  large  percentage  of  the  sugar  absorbed  is  at  once  oxidized.  The 
reduction  of  fat  and  sugar  to  carbon  dioxid  and  water,  under  which 
forms  they  are  eliminated  from  the  body,  is  accompanied  by  a  dis- 
engagement of  a  large  quantity  of  heat. 

Water  is  present  in  all  the  fluids  and  solids  of  the  body.  It  pro- 
motes the  absorption  of  new  material  from  the  alimentary  canal ;  it 
holds  the  various  ingredients  of  the  blood,  lymph,  and  other  fluids 
in  solution ;  it  hastens  the  absorption  of  waste  products  from  the 
tissues,  and  promotes  their  speedy  elimination  from  the  body. 

Sodium  chlorid  is  present  in  all  parts  of  the  body  to  the  extent 
of  no  gm.  The  average  amount  eliminated  daily  is  15  gm.  Its 
necessity  as  an  article  of  diet  is  at  once  apparent.  Taken  as  a  con- 
diment, it  imparts  sapidity  to  the  food,  excites  the  flow  of  the  di- 
gestive fluids,  promotes  the  absorption  and  assimilation  of  the  al- 
bumins, influences  the  passage  of  nutritive  material  through  animal 
membranes,  and  furnishes  the  chlorin  for  the  free  hydrochloric  acid 
of  the  gastric  juice.  In  some  unknown  way  it  favorably  promotes  the 
activity  of  the  general  nutritive  process. 

The  potassium  salts  are  also  essential  to  the  normal  activity  of 
the  nutritive  process.  When  deprived  of  these  salts,  animals  become 
weak  and  emaciated.  When  given  in  small  doses,  they  increase  the 
force  of  the  heart-beat,  raise  the  arterial  pressure,  and  thus  increase 
the  action  of  the  circulation  of  the  blood. 

The  calcium  phosphate  and  carbonate  are  utilized  in  imparting 
solidity  to  the  tissues,  more  especially  the  bones  and  teeth.  Many 
articles  of  food  contain  these  salts  in  quantities  sufficient  to  restore 
the  amount  lost  daily. 

The  vegetable  acids  increase  the  secretions  of  the  alimentary  canal, 
and  are  apt,  in  large  amounts,  to  produce  flatulence  and  diarrhea. 
After  entering  into  combination  with  bases  to  form  salts,  they  stimu- 
late the  action  of  the  kidneys  and  promote  a  greater  elimination  of 
all  the  urinary  constituents.  In  some  unknown  way  they  influence 
nutrition ;  when  deprived  of  these  acids,  the  individual  becomes 
scorbutic. 


FOODS    AND   DIETETICS.  91 

The  accessory  foods,  coffee  and  tea,  when  taken  in  moderation, 
overcome  the  sense  of  fatigue  and  mental  unrest  consequent  on  exces- 
sive physical  and  mental  exertion.  Coffee  increases  the  action  of  the 
intestinal  glands  and  acts  as  a  laxative.  After  absorption,  its  active 
principle,  caffein,  stimulates  the  action  of  the  heart,  raises  the 
arterial  pressure,  and  excites  the  action  of  the  brain.  Tea  acts  as 
an  astringent,  owing  to  the  tannic  acid  it  contains.  One  effect  of 
the  tannic  acid  is  to  coagulate  the  digestive  ferments  and  to  interfere 
with  the  activity  of  the  digestive  process. 

Alcohol,  when  introduced  into  the  system  in  small  quantities,  under- 
goes oxidation  and  contributes  to  the  production  of  force,  and  is 
thus  far  a  food.  It  excites  the  gastric  glands  to  increased  secretion, 
improves  the  digestion,  accelerates  the  action  of  the  heart,  and  stimu- 
lates the  activities  of  the  nerve  centers.  In  zymotic  diseases,  and  in 
all  cases  of  depression  of  the  vital  powers,  it  is  most  useful  as  a 
restorative  agent.  When  taken  in  excessive  quantities,  it  is  elimi- 
nated by  the  lungs  and  kidneys.  The  metamorphosis  of  the  tissue  is 
retarded,  the  elimination  of  urea  and  carbonic  acid  is  lessened,  the 
temperature  is  lowered,  the  muscular  powers  are  impaired,  and  the 
resistance  to  depressing  external  influences  is  diminished.  When 
taken  throughout  a  long  period  of  time,  alcohol  impairs  digestion, 
produces  gastric  catarrh,  and  disorders  the  secreting  power  of  the 
hepatic  cells.  It  also  diminishes  the  muscular  power  and  destroys 
the  structure  and  composition  of  the  cells  of  the  brain  and  spinal 
cord.  The  connective  tissue  of  the  body  increases  in  amount,  and, 
subsequently  contracting,  gives  rise  to  sclerosis. 

A  proper  combination  of  various  alimentary  principles  is  essential 
for  healthy  nutrition,  no  one  class  being  capable  of  maintaining  life 
for  any  definite  length  of  time. 

The  albuminous  food  in  excess  promotes  the  arthritic  diathesis, 
manifesting  itself  as  gout,  gravel,  etc. 

The  oleaginous  food  in  excess  gives  rise  to  the  bilious  diathesis, 
while  a  deficiency  of  it  promotes  the  scrofulous. 

The  farinaceous  food  when  long  continued  in  excess,  favors  the 
rheumatic  diathesis  by  the  development  of  lactic  acid. 

The  quantities  of  the  different  nutritive  materials  which  are 
required  daily  for  the  growth  and  repair  of  the  tissues  and  for  the 
evolution  of  heat  have  been  variously  estimated  by  different  ob- 
servers. The  following  table  shows  the  average  diet  scale  of  Vierbrdt, 
and  the  amount  of  waste  products  to  which  it  would  give  rise : 


92  HUMAN   PHYSIOLOGY. 

COMPARISON  OF  THE  INGESTA  AND  EGESTA. 

Ingesta.  Egesta. 

Proteids    .     .     .      120  grams.  Urea     ....        40  grams. 

Fat        ....        90  Inorganic    salts.        32 

Starch       .     .     .      330     ."  Feces    ....      104      " 

Inorganic    salts.        32       "  Carbon  dioxid  .      800       " 

Water        .     .     .  2,800       "  Water        .     .     .  3,096       " 
Oxygen     .     .     .      700       "  Total      .     ."4^72       " 

Total      .     .  4,072       " 

Other  estimates  as  to  the  amount  of  the  organic  substances  required 
daily  are  as  follows : 


Ranke. 

Voit. 

Atwater. 

Moleschoit. 

Proteid     . 

.    100 

118 

125 

130  grams. 

Fat       .     . 

.     100 

5o 

125 

84       " 

Starch       . 

.  240 

500 

400 

404 

The  Energy  of  the  Animal  Body. — The  food  consumed  daily  not 
only  repairs  the  loss  of  material  from  the  body,  but  also  furnishes 
the  energy  to  replace  that  which  is  expended  daily  in  the  shape  of 
heat  and  motion.  All  the  energy  of  the  body  can  be  traced  to  the 
chemic  changes  going  on  in  the  tissues,  and  more  particularly  to  those 
changes  involved  in  the  oxidation  of  the  foods. 

The  amount  of  heat  yielded  by  any  given  food  principle  can  be 
determined  by  burning  it  to  carbon  dioxid  and  water,  and  ascertain- 
ing the  extent  to  which  it  will,  when  so  liberated,  raise  the  tempera- 
ture of  a  given  volume  of  water.  This  amount  of  heat  may  be 
expressed  in  calories.  A  calorie  is  the  amount  of  heat  required  to 
raise  the  temperature  of  one  kilogram  of  water  one  degree  Centi- 
grade. 

The  following  estimates  give,  approximately,  the  number  of  calories 
produced  when  the  food  is  reduced  within  the  body  to  urea,  carbon 
dioxid,  and  water : 

i  gram  of  proteid  yields  4,124  kilogram  calories, 
i        "        fat  "     9,353 

i        "        starch  4,n6 

The  total  number  of  kilogram  calories  yielded  by  any  given  diet 
scale  can  be  readily  determined  by  multiplying  the  preceding  factors 


FOODS    AND   DIETETICS.  93 

by  the  quantities  of  material  consumed.     The  diet  scale  of  Ranke,  for 
example,  yields  the  following  amount : 

100  grams  of  proteid  yield  412.4  calories. 
100  "       fat  "    935-3 

240  "       starch         "    987.8 


Total 2,335.5 

It  has  also  been  determined  experimentally  that  one  gram  of 
proteid,  one  gram  of  fat,  and  one  gram  of  starch,  when  completely 
oxidized,  will  yield  energy  sufficient  to  perform,  1,850,  3,841,  and 
1,567  kilogrammeters  of  work,  respectively.  A  kilogrammeter  of 
work  is  one  kilogram  raised  one  meter  high. 

The  total  energy  of  the  Ranke  diet  scale  can  be  easily  calculated 
—e.  £., 

100  grams  of  proteid  yield  185,000  kilogrammeters. 

100          "        fat  '    384,100 

240          "        starch         "     397,680 


Total       .     .     .     .     .  966,780 

It  will  be  thus  seen  that  the  food  consumed  daily  yields  2,335 
kilogram  calories,  which  can  be  translated  into  its  mechanical  equiva- 
lent, 966,780  kilogrammeters  of  work. 

The  amount  of  food  required  in  twenty-four  hours  is  estimated 
from  the  total  quantity  of  carbon  and  nitrogen  excreted  from  the 
body  in  twenty-four  hours,  these  two  elements  representing  the 
waste  or  destruction  of  the  carbonaceous  and  nitrogenized  compounds. 
It  has  been  determined  by  experimentation  that  about  4,600  grains 
of  carbon  and  about  300  grains  of  nitrogen  are  eliminated  from  the 
body  daily,  the  ratio  being  about  15  to  i.  That  the  body  may  be 
kept  in  its  normal  condition,  a  proper  proportion  of  carbonaceous 
(bread)  to  nitrogenized  (meat)  food  should  be  observed  in  the  diet. 

The  method  of  determining  the  proper  amounts  of  both  kinds  of 
food  is  as  follows  : 

1,000  grs.  of  bread  (2  oz.)  contain  300  grs.  C.  and  10  grs.  N. 

To  obtain  the  requisite  amount  of  nitrogen  from  bread,  30,000 
grains,  or  about  four  pounds,  containing  9,000  grains  of  carbon  and 


94  HUMAN   PHYSIOLOGY. 

300  of  nitrogen,  would  have  to  be  consumed.  On  such  a  diet  there 
would  be  a  large  excess  of  carbon,  which  would  be  undesirable.  On 
a  meat  diet  the  reverse  obtains : 

1,000  grs.  of  meat  (2  oz.)  contain  100  grs.  C  and  30  grs.  N. 

To  obtain  the  requisite  amount  of  carbon  from  meat,  45,000  grains, 
or  about  6l/2  pounds,  containing  4,500  grains .  of  carbon  and  1,350 
grains  of  nitrogen  would  have  to  be  consumed.  Under  such  cir- 
cumstances there  would  arise  an  excess  of  nitrogen  in  the  system, 
which  would  be  equally  undesirable  and  injurious.  By  combining 
these  two  articles,  however,  in  proper  proportion,  the  requisite 
amounts  of  carbon  and  nitrogen  can  be  obtained  without  any  excess 
of  either — e.  g. : 


2  pounds  of  bread  contain  4,630  grs.  C  and  154  grs. 
V*          "         meat  "          463          "          154 


5,093  C.  308  N. 

The  amount  of  carbon  and  nitrogen  necessary  to  compensate  for 
the  loss  to  the  system  daily  would  be  contained  in  the  foregoing 
amount  of  food.  As  about  3^  ounces  of  oil  or  butter  are  consumed 
daily,  the  quantity  of  bread  can  be  reduced  to  19  ounces.  In  the 
quantities  of  bread  and  meat  just  mentioned  there  are  4.2  ounces 
albumin,  9.3  sugar  and  starch. 

The  alimentary  principles  are  not  introduced  into  the  body  as  such, 
but  are  combined  in  proper  proportions  to  form  compound  sub- 
stances, termed  foods, — e.  g.,  bread,  milk,  eggs,  meat,  etc., — the 
nutritive  value  of  each  depending  upon  the  extent  to  which  these 
principles  exist. 

The  following  tables  show  the  average  composition  of  various 
articles  of  food : 


FOODS    AND  DIETETICS. 

COMPOSITION  OF  ANIMAL  FOODS. 


95 


IN  100  PARTS. 

BEEF. 

VEAL. 

MUTTON. 

PORK. 

FOWL. 

FISH. 

Water,     .... 

76.25 

77-82 

75-59 

72.57 

70.80 

79-30 

Proteid,   .... 

20.24 

19.86 

17.11 

I9-31 

22.70 

18.30 

Fat 

I  68 

o  82 

C  47 

*  82 

4  10 

o  70 

Carbohydrates,    . 

0.50 

0.80 

O.6O 

0.60 

1.20 

0.90 

Salts,   .... 

i  38 

o  no 

I    21 

I   7O 

I   20 

o  80 

COMPOSITION    OF   VEGETABLE   FOODS. 


IN  zoo  PARTS. 

BEANS. 

PEAS. 

POTA- 
TOES 

TURNIPS. 

CABBAGE. 

ASPARA- 
GUS. 

Water,     .... 

13.74 

14.  QQ 

7<?  47 

80  42 

80  Q7 

Q3  7C 

yj'  /;> 

Proteid,   .... 

23.21 

22.85 

1.95 

i-35 

1.89 

1.79 

Fat,      .    .    . 

2   Id. 

I   7Q 

o  i  c 

o  18 

o  20 

o  ?  c 

V.  1^ 

0.^5 

Carbohydrates,  . 

53-67 

52,36 

20.69 

7.36 

4.87 

2.63 

Cellulose,    .    .    . 

3-69 

5-43 

0.76 

0-94 

1.84 

1.04 

Salts,    ..... 

•5.  Cf 

2  s8 

o  98 

O  7£ 

T    "21 

OC/I 

u«  /O 

L-^6 

•54 

96  HUMAN    PHYSIOLOGY. 

COMPOSITION  OF  CEREAL  FOODS. 


IN  100  PARTS. 

WHEAT. 

RYE. 

BARLEY. 

OATS. 

CORN. 

RICE 

Water,     ..... 

I3-56 

12.65 

13-77 

12.37 

13.10 

I3   12 

Proteid,   .... 

12.35 

12.55 

11.14 

10.41 

9.85 

7.88 

Fat,      

1.7? 

I.Q7 

2.16 

?.2^ 

4.^7 

0.85 

Carbohydrates,    . 

67.90 

67.95 

64.93 

57-78 

68.42 

76.55 

Cellulose,    .    .    . 

2.63 

300 

5-31 

11.19 

2.50 

0-55 

Salts,  

i  81 

i  88 

2  6Q 

3.  02 

1.56 

1.05 

DIGESTION. 

Digestion  is  a  physical  and  chemic  process  by  which  the  food  intro- 
duced into  the  alimentary  canal  is  liquefied  and  its  nutritive  prin- 
ciples are  transformed  by  the  digestive  fluids  into  new  substances 
capable  of  being  absorbed  into  the  blood. 

The  digestive  apparatus  consists  of  the  alimentary  canal  and  its 
appendages — viz.,  teeth ;  salivary,  gastric  and  intestinal  glands ;  liver  ; 
and  pancreas. 

Digestion  may  be  divided  into  seven  stages:  prehension,  mastica- 
tion, insalivation,  deglutition,  gastric  and  intestinal  digestion,  and 
defecation. 

Prehension,  the  act  of  conveying  food  into  the  mouth,  is  accom- 
plished by  the  hands,  lips,  and  teeth. 

MASTICATION. 

Mastication  is  the  trituration  of  the  food,  and  is  accomplished 
by  the  teeth  and  lower  jaw  under  the  influence  of  muscular  contrac- 


DIGESTION.  97 

tion.  When  thoroughly  divided,  the  food  presents  a  larger  surface 
for  the  solvent  action  of  the  digestive  fluids,  thus  aiding  the  general 
process  of  digestion. 

The  teeth  are  thirty-two  in  number,  sixteen  in  each  jaw,  and 
divided  into  four  incisors  or  cutting  teeth,  two  canines,  four  bicuspids, 
and  six  molars  or  grinding  teeth ;  each  tooth  consists  of  a  crown 
covered  by  enamel,  a  neck,  and  a  root  surrounded  by  the  crusta 
petrosa  and  embedded  in  the  alveolar  process ;  a  section  through  a 
tooth  shows  that  its  substance  is  made  of  dentine,  in  the  center  of 
which  is  the  pulp  cavity  containing  blood-vessels  and  nerves. 

The  loiver  jaw  is  capable  of  making  a  downward  and  an  upward, 
a  lateral  and  an  anteroposterior  movement,  dependent  upon  the  c6n- 
struction  of  the  temporomaxillary  articulation. 

The  jaw  is  depressed  by  the  contraction  of  the  digastric,  geniohyoid, 
mylohyoid,  and  platysma  myoides  muscles ;  elevated  by  the  temporal, 
mas  set  er,  and  internal  pterygoid  muscles  ;  moved  laterally  by  the  alter- 
nate contraction  of  the  external  pterygoid  muscles ;  moved  anteriorly 
by  the  pterygoid,  and  posteriorly  by  the  united  actions  of  the  genio- 
hyoid, mylohoid,  and  posterior  fibers  of  the  temporal  muscles. 

The  food  is  kept  between  the  teeth  by  the  intrinsic  and  extrinsic 
muscles  of  the  tongue  from  within,  and  the  orbicularis  oris  and 
buccinator  muscles  from  without. 

The  movements  of  mastication,  though  originating  in  an  effort  of 
the  will  and  under  its  control,  are,  for  the  most  part,  of  an  automatic 
or  reflex  character,  taking  place  through  the  medulla  oblongata  and 
induced  by  the  presence  of  food  within  the  mouth.  The  nerves  and 
nerve-centers  involved  in  this  mechanism  are  shown  in  the  follow- 
ing table : 

Nerve  Mechanism  of  Mastication. 
A  fferent  Nerves.  Efferent  Nerves. 

1.  Lingual  branch  of  5th  pair.        i.  Third  branch  of  5th  pair. 

2.  Glossopharyngeal.  2.  Hypoglossal 

3.  Facial. 

The  impressions  made  upon  the  terminal  filaments  of  the  afferent 
nerves  are  transmitted  to  the  medulla ;  motor  impulses  are  here  gen- 
erated which  are  transmitted  through  motor  nerves  to  the  muscles 
involved  in  the  movements  of  the  lower  jaw.  The  medulla  not  only 
generates  motor  impulses,  but  coordinates  them  in  such  a  manner 
8 


98 


HUMAN    PHYSIOLOGY. 


that    the    movements    of    mastication    may    be    directed    toward    the 
accomplishment  of  a  definite  purpose. 

INSALIVATION. 

Insalivation  is  the  incorporation  of  the  food  with  the  saliva 
secreted  by  the  parotid,  submaxillary,  and  sublingual  glands;  the 
parotid  saliva,  thin  and  watery,  is  poured  into  the  mouth  through 
Steno's  duct ;  the  submaxillary  and  sublingual  salivas,  thick  and 
viscid,  are  poured  into  the  mouth  through  Wharton's  and  Bartholin's 
ducts. 

In  their  minute  structure  the  salivary  glands  resemble  one  another. 
They  belong  to  the  racemose  variety,  and  consist  of  small  sacs 
or  vesicles,  which  are  the  terminal  expansions  of  the  smallest  sali- 
vary ducts.  Each  vesicle  or  acinus  consists  of  a  basement  mem- 
brane surrounded  by  blood-vessels  and  lined  with  epithelial  cells. 
In  the  parotid  gland  the  lining  cells  are  granular  and  nucleated  ;  in 
the  submaxillary  and  sublingual  glands  the  cells  are  large,  clear, 


FIG.  ii. — CELLS  OF  THE  ALVEOLI  OF.  A  SEROUS  OR  WATERY  SALIVARY  GLAND. — 
(Yeo's  "Text-book  of  Physiology.") 

A.  After  rest.     B.  After  a  short  period  of  activity.      C.  After  a  prolonged 
period  of  activity. 

and  contain  a  quantity  of  mucigen.  During  and  after  secretion  very 
remarkable  changes  take  place  in  the  cells  lining  the  acini,  which  are 
in  some  way  connected  with  the  essential  constituents  of  the  sali- 
vary fluids. 

In  the  living  serous  gland, — e.  g.,  parotid, — during  rest,  the  secre- 
tory cells  lining  the  acini  of  the  gland  are  seen  to  be  filled  with  fine 
granules,  which  are  often  so  abundant  as  to  obscure  the  nucleus  and 
enlarge  the  cells  until  the  lumen  of  the  acinus  is  almost  obliterated. 
(See  Fig.  n.)  When  the  gland  begins  to  secrete  the  saliva,  the 


DIGESTION.  yy 

granules  disappear  from  the  outer  boundary  of  the  cells,  which 
then  become  clear  and  distinct.  At  the  end  of  the  secretory  activity 
the  cells  have  been  freed  of  granules  and  have  become  smaller  and 
more  distinct  in  outline.  It  would  seem  that  the  granular  matter 
is  formed  in  the  cells  during  the  period  of  rest  and  discharged  into 
the  ducts  during  the  activity  of  the  gland. 

In    the    mucous    glands — e.    g.,    submaxillary    and    sublingual — the 
changes  that  occur  in  the  cells  are  somewhat  different.     (See  Fig.  12.) 


FIG.     12. — SECTION    OF    A    Mucous    GLAND. — (Lavdowsky,) 
A.  In  a  state  of  rest.     B.  After  it  has  been  for  some  time  actively  secreting. 

During  the  intervals  of  digestion  the  cells  lining  the  gland  are  large, 
clear,  and  highly  refractive,  and  contain  a  large  quantity  of  mucigin. 
After  secretion  has  taken  place  the  cells  exhibit  a  marked  change. 
The  mucigin  cells  have  disappeared,  and  in  their  place  are  cells 
which  are  small,  dark,  and  composed  of  protoplasm.  It  would  ap- 
pear that  the  cells,  during  rest,  elaborate  the  mucigin,  which  is  dis- 
charged into  the  tubules  during  secretory  activity,  to  become  part 
of  the  secretion. 

Saliva  is  an  opalescent,  slightly  viscid,  alkaline  fluid,  having  a 
specific  gravity  of  1.005.  Microscopic  examination  reveals  the  pres- 
ence of  salivary  corpuscles  and  epithelial  cells.  Chemically  it  is 
composed  of  water,  proteid  matter,  a  ferment  (ptyalin),  and  inor- 
ganic salts.  The  amount  secreted  in  twenty-four  hours  is  about  2^ 
Ibs.  Its  function  is  twofold  : 
i.  Physical. — Softens  and  moistens  the  food,  agglutinates  it,  and 

facilitates   swallowing. 


100  HUMAN    PHYSIOLOGY. 

2.  Chemic. — Converts  starch  into  sugar.  This  action  is  due  to  the 
presence  of  the  organic  ferment,  ptyalin.  Ptyalin  is  an  amorphous 
nitrogenized  substance,  which  can  be  precipitated  from  the  saliva 
by  calcium  phosphate.  Its  power  of  converting  starch  into  sugar 
is  manifested  most  decidedly  at  the  temperature  of  the  living 
body  and  in  a  slightly  alkaline  medium.  The  conversion  of  starch 
into  sugar  takes  place  through  several  stages,  the  nature  of  which 
depends  upon  the  structure  of  the  starch  granule.  This  consists  of 
two  portions,  a  stroma  of  cellulose  and  a  contained  material, 
granulose,  which  is  the  more  abundant  and  important  of  the 
two.  When  subjected  to  the  action  of  boiling  water,  the  starch 
granule  swells  up  and  bursts,  forming  a  viscid,  opalescent  mass  of 
starch  paste.  If  saliva  be  now  added  to  this  paste  and  kept  at  a 
temperature  of  104°  F.  for  a  few  minutes,  the  paste  becomes 
clear  and  limpid.  The  first  stage  in  the  digestion  is  now  com- 
plete, with  the  formation  of  soluble  starch.  If  the  action  of  saliva 
be  continued,  a  number  of  substances  intermediate  between  starch 
and  sugar  are  formed,  to  which  the  name  dextrin  has  been  given. 
Among  these  may  be  mentioned : 

(a)   Erythrodextrin,   which   gives   the   reddish-brown    color   with 

iodin.      As   the   digestion   continues   and   sugar  is   formed,   the 

erythrodextrin  disappears,  giving  way  to — 
(fr)   Achroodextrin,   which   yields   no   coloration   with   iodin,   but 

which  may  be  precipitated  by  alcohol. 

The  sugar  formed  by  the  action  of  saliva  is  maltose,  the  formula 
for  which  is  C^H^On.  A  small  quantity  of  dextrose  is  also  formed. 

NERVE  MECHANISM  OF  INSALIVATiON. 

Afferent  Nerves.  Efferent  Nerves. 

1.  Lingual  branch  of  5th  pair.  i.   Auriculotemporal  branch  of  5th 

2.  Glossopharyngeal.  pair,  for  parotid  gland. 

2.   Chorda  tympani,  for  submaxil- 
lary  and  sublingual  glands. 

The  centers  regulating  the  secretion  are  located  in  the  medulla 
oblongata.  The  nerve  mechanism  is  excited  to  action  by  nerve  im- 
pulses developed  by  the  contact  of  the  food  with  the  terminal  fila- 
ments of  afferent  nerves  in  the  mucous  membrane  of  the  mouth. 
These  nerve  impulses  are  transmitted  by  afferent  nerves  to  the  med- 


DIGESTION.  101 

ulla  oblongata  where  they  are  transformed  into  motor  impulses 
which  are  then  transmitted  through  efferent  nerves  to  the  blood- 
vessels and  epithelium  of  the  glands. 

Stimulation  of  the  auriculotemporal  branch  increases  the  flow  of 
saliva  from  the  parotid  gland ;  division  arrests  it. 

Stimulation  of  the  chorda  tympani  is  followed  by  a  dilatation  of 
the  blood-vessels  of  the  submaxillary  and  sublingual  glands,  an  in- 
creased flow  of  blood  and  an  abundant  discharge  of  a  thin  saliva ; 
division  of  the  nerve  arrests  the  secretion. 

Stimulation  of  the  cervical  sympathetic  is  followed  by  a  contrac- 
tion of  the  blood-vessels,  a  diminished  flow  of  blood,  and  a  diminu- 
tion of  the  secretion,  which  now  becomes  thick  and  viscid ;  division 
of  the  sympathetic  is  not,  however,  followed  by  complete  dilatation 
of  the  vessels.  There  is  evidence  of  the  existence  of  a  local  vaso- 
motor  mechanism,  which  is  inhibited  by  the  chorda  tympani,  and 
augmented  by  the  sympathetic. 

DEGLUTITION. 

Deglutition  is  the  act  of  transferring  food  from  the  mouth  into 
the  stomach,  and  may  be  divided  into  three  stages  : 

1.  The  passage  of  the  bolus  from  the  mouth  into  the  pharynx. 

2.  From  the  pharynx  into  the  esophagus. 

3.  From  the  esophagus  into  the  stomach. 

In  the  first  stage,  which  is  entirely  voluntary,  the  mouth  is  closed 
and  respiration  momentarily  suspended ;  the  tongue,  placed  against 
the  roof  of  the  mouth,  arches  upward  and  backward,  and  forces  the 
bolus  into  the  fauces. 

In  the  second  stage,  which  is  entirely  reflex,  the  palate  is  made 
tense  and  directed  upward  and  backward  by  the  levatores  palati  and 
tensores  palati  muscles ;  the  bolus  is  grasped  by  the  superior  con- 
strictor muscle  of  the  pharynx  and  rapidly  forced  into  the  esophagus. 

The  food  is  prevented  from  entering  the  posterior  nares  by  the 
uvula  and  the  closure  of  the  posterior  half-arches  (the.  palatopharyn- 
geal  muscles)  ;  from  entering  the  larynx  by  its  ascent  under  the  base 
of  the  tongue  and  the  action  of  the  epiglottis. 

In  the  third  stage  the  longitudinal  and  circular  muscle-fibers,  con- 
tracting from  above  downward,  strip  the  bolus  into  the  stomach. 


102  HUMAN   PHYSIOLOGY. 

GASTRIC    DIGESTION. 

The  Stomach. — Immediately  beyond  the  termination  of  the  esopha- 
gus the  alimentary  canal  expands  and  forms  a  receptacle  for  the 
temporary  retention  of  the  food.  To  this  dilatation  the  term  stomach 
has  been  applied.  This  organ  is  somewhat  pyriform  in  outline,  and 
occupies  the  upper  part  of  the  abdominal  cavity.  It  is  about  13 
inches  long,  5  deep,  and  3^  wide,  and  has  a  capacity  of  about  five 
pints.  It  presents  two  orifices,  the  cardiac  or  esophageal,  and  the 
pyloric  ;  two  curvatures,  the  lesser  and  the  greater. 

The  left  or  cardiac  end  of  the  stomach  is  enlarged,  and  forms  the 
fundus  ;  the  right  end  is  much  narrower,  and  forms  the  pylorus.  The 
stomach  possesses  three  coats : 

1.  The  serous,  or  reflection  of  the  peritoneum. 

2.  The  muscular,  the  fibers  of  which  are  arranged  in  a  longitudinal, 
a    circular,    and    an    oblique    direction.      At    the    pyloric    end    the 
circular  fibers  increase  in  number  and  form  a  thick  ring  or  band, 
which  is  known  as  the  sphincter  of  the  pylorus. 

3.  The  mucous,  which  is  somewhat  larger  than  the  muscular  coat,  and 
in  consequence  is  thrown  into  folds  or  rugae.     The  surface  of  the 
mucous  coat  is  covered  by  tall,  narrow,  columnar  epithelium. 

Gastric  Juice. — During  the  period  of  time  the  food  remains  in  the 
stomach  it  is  subjected  to  the  disintegrating  action  of  an  acid  fluid, 
the  gastric  juice.  This  fluid,  secreted  from  glands  in  the  mucous 
membrane,  is  thoroughly  incorporated  with  the  food  in  consequence 
of  the  contractions  of  the  muscular  coat.  The  food  is  gradually 
liquefied  and  reduced  to  a  form  which  partly  fits  it  for  passage  into 
the  small  intestine  and  for  absorption  into  the  blood.  Gastric  juice, 
when  obtained  in  a  pure  state,  is  a  clear,  colorless  fluid,  decidedly  acid 
in  reaction,  with  a  specific  gravity  of  1005.  It  is  composed  of  the 
following  ingredients  : 

COMPOSITION    OF    GASTRIC    JUICE. 

Water 994.404 

Hydrochloric  acid 0.200 

Organic   matter 3-195 

Inorganic    salts 2.201 


DIGESTION. 


103 


The  water  forms  by  far  the  largest  part  of  this  fluid,  and  serves 
the  purpose  of  holding  the  other  ingredients  in  solution,  and  by  its 
saturating  power  brings  them  into  relation  with  the  constituents  of 
the  food.  Of  the  inorganic  salts  the  sodium  and  potassium  chlorids 
are  the  most  abundant  and  important. 

The  hydrochloric  acid,  which  exists  in  a  free  state,  is  present 
in  variable  amounts.  In  the  foregoing  table  the  number  of  parts  a 
thousand  is  much  smaller  than  is  usually  stated.  According  to  most 
observers,  hydrochloric  acid  is  present  to  the  extent  of  from  0.2  to 
0.3  part  a  hundred.  Though  secreted  as  soon  as  the  food  enters  the 
stomach,  the  acid  can  not  be  detected  in  the  free  state  until  after  the 
lapse  of  from  thirty  to  forty  minutes.  It  acidulates  the  food  and 
prevents  fermentative  changes. 

The  pepsin,  which  is  present  in  gastric  juice  associated  with  the 
organic  matter,  is  a  hydrolytic  ferment  or  enzyme.  When  freed 
from  its  associations  and  obtained  in  a  pure  state,  pepsin  presents  the 
characteristics  of  a  colloid  body,  and  resembles  in  its  reactions  the 
albuminoids.  It  has  the  power,  when  brought  into  relation  with 
acidulated  proteids,  of  transforming  them  into  new  forms  capable  of 
absorption  into  the  blood. 

Rennin. — In  addition  to  pepsin  a  second  ferment  exists  in  the 
gastric  juice,  to  which  the  term  rennin  has  been  given.  It  possesses 
the  power  of  coagulating  the  caseinogen  of  milk.  It  exists  in  the 
mucous  membrane,  from  which  it  can  be  extracted  by  appropriate 
means.  When  rennin  acts  on  caseinogen,  the  latter  is  split  into 
insoluble  casein  and  a  soluble  albumin.  Calcium  phosphate  is  essen- 
tial to  the  action  of  this  enzyme. 

Gastric  Glands. — Embedded  within  the  mucous  membrane  are  to 
be  found  enormous  numbers  of  tubular  glands,  which  though  resem- 
bling one  another  in  general  form,  differ  in  their  histologic  details 
in  various  portions  of  the  stomach. 

In  the  cardiac  end  or  fundus  the  glands  consist  of  several  long 
tubules,  opening  into  a  short,  common  duct,  which  opens  by  a  wide 
mouth  on  the  surface  of  the  mucous  membrane.  Each  gland  consists 
primarily  of  a  basement  membrane  lined  by  epithelial  cells.  In  the 
duct  the  epithelium  is  of  the  columnar  variety,  resembling  that 
covering  the  surface  of  the  mucous  membrane.  The  secretory  por- 
tion of  the  tubule  is  lined  by  a  layer  of  short,  polyhedral,  granular, 
and  nucleated  cells,  which,  as  they  border  the  lumen  of  the  tubule, 


104 


HUMAN   PHYSIOLOGY. 


and  thus  occupy  the  central  portion  of  the  gland,  are  termed  central 
cells.  At  irregular  intervals,  between  the  central  cells  and  the  wall 
of  the  tubule,  are  found  large,  oval,  reticulated  cells,  which,  on  ac- 
count of  their  position,  are  termed  parietal  cells.  (See  Fig.  13.) 


FIG.    13. 

.  Diagram  showing  the  relation  of  the  ultimate  twigs  of  the  blood-vessels,  V. 
and  A,  and  of  the  absorbent  radicles  to  the  glands  of  the  stomach  and  the 
different  kinds  of  epithelium — viz.,  above  cylindric  cells;  small,  pale  cells 
in  the  lumen,  outside  which  are  the  dark  ovoid  cells. —  (Yeo's  "Text-book 
of  Physiology.") 

Each  parietal  cell  is  in  relation  with  a  system  of  fine  canals,  which 
open  directly  into  the  lumen  of  the  gland.  It  is  estimated  that  the 
fundus  contains  about  five  million  glands.  In  the  pyloric  end  of 
the  stomach  the  glands  are  generally  branched  at  their  lower  ex- 
tremities, and  the  common  duct  is  long  and  wide.  The  duct  is  lined 
by  columnar  epithelium,  while  the  secreting  part  is  lined  by  short, 
slightly  columnar,  granular  cells.  The  parietal  cells  are  entirely 
wanting.  The  epithelium  covering  the  surface  of  the  mucous  mem- 
brane is  tall,  narrow  and  cylindric  in  shape,  and  consists  of  mucus- 
secreting  goblet  cells.  The  outer  half  of  the  cell  contains  a  sub- 


DIGESTION.  105 

stance,  mucinogen,  which  produces  mucin.  The  gastric  glands  in 
both  situations  are  surrounded  by  a  fine  connective  tissue,  which  sup- 
ports blood-vessels,  nerves,  and  lymphatics. 

Changes  in  the  Cells  During  Secretion. — During  the  periods  of 
rest  and  secretory  activity  the  cells  of  the  glands  undergo  changes  in 
structure  which  are  supposed  to  be  connected  with  the  production 
of  the  pepsin  and  hydrochloric  acid.  During  rest,  the  protoplasm 
of  the  central  cells  becomes  filled  with  granular  matter;  during  the 
time  of  secretion  this  disappears,  presumably  passing  into  the  lumen 
of  the  tubule,  and  as  a  result  the  protoplasm  becomes  clear  and 
hyalin  in  appearance.  The  granular  material  is  probably  the  mother 
substance,  pepsinogen,  which,  inactive  in  itself,  yields  the  active 
ferment,  pepsin.  The  parietal  cells  during  digestion  increase  in 
size,  but  do  not  become  granular.  It  is  at  this  period  that  they 
secrete  the  hydrochloric  acid.  After  digestion  they  rapidly  diminish 
in  size  and  return  to  their  former  condition.  The  pyloric  glands 
secrete  pepsin  only. 

Mechanism  of  Secretion. — In  the  intervals  of  digestion,  the  mucous 
membrane  of  the  stomach  is  covered  with  a  layer  of  mucus.  As 
soon  as  the  food  passes  from  the  esophagus  into  the  stomach,  the 
blood-vessels  dilate,  the  circulation  becomes  more  active,  and  Jihe 
membrane  assumes  a  bright  red  appearance.  Coincidentally,  small 
drops  of  gastric  juice  begin  to  exude  from  the  glands,  which  as  they 
increase  in  number,  run  together  and  trickle  down  the  sides  of  the 
stomach.  This  pouring  out  of  fluid  continues  during  the  presence  of 
food  in  the  stomach. 

The  secretion  of  gastric  juice  is  a  reflex  act,  taking  place  through 
the  central  nervous  system  and  called  forth  in  response  to  the  stimulus 
of  food  in  the  stomach.  That  the  central  nervous  system  also  directly 
influences  the  production  of  the  secretion  is  shown  by  the  fact  that 
emotion,  such  as  fear  or  anger,  will  arrest  or  vitiate  the  normal  secre- 
tion. The  reflex  nature  of  the  process  can  be  shown  by  experimenta- 
tion upon  the  pneumogastric  nerve.  If  during  digestion,  when  the 
peristaltic  movements  are  active  and  the  gastric  mucous  membrane 
is  flushed  and  covered  with  gastric  juice,  the  pneumogastric  nerves 
are  divided  on  both  sides,  the  mucous  membrane  becomes  pale,  the 
secretion  is  arrested,  and  the  peristaltic  movements  become  less 
marked.  Stimulation  of  the  peripheral  end  produces  no  constant 
effects ;  stimulation  of  the  central  end,  however,  is  at  once  fol- 


106  HUMAN   PHYSIOLOGY. 

lowed  by  dilatation  of  the  vessels,  flushing  of  the  mucous  membrane, 
and  reestablishment  of  the  secretion.  It  is  evident,  therefore,  that 
during  digestion  afferent  impulses  are  passing  up  the  pneumogastrics 
to  the  medulla;  efferent  impulses,  in  all  probability,  pass  through 
the  fibers  of  the  sympathetic  nervous  system  to  the  blood-vessels 
and  glands  concerned  in  the  elaboration  of  the  gastric  juice.  After 
all  the  nerve  connections  of  the  stomach  are  divided,  the  secretion 
of  a  small  quantity  of  juice  continues  for  several  days.  This  has 
been  attributed  to  the  action  of  a  local  nervous  mechanism  and  to 
the  direct  action  of  the  food  upon  the  protoplasm  of  the  secreting 
cells. 

Chemic  Action  of  the  Gastric  Juice. — By  the  combined  influence 
of  the  contraction  of  the  muscular  walls,  the  action  of  the  gastric 
juice,  and  the  temperature,  the  food  is  reduced  to  a'  semiliquid  con- 
dition and  acquires  a  distinct  acid  odor.  This  semifluid  mass  will  vary 
in  composition  and  appearance  according  to  the  nature  of  the  food. 
To  this  matter  the  term  chyme  has  been  given. 

Meat  is  rapidly  disintegrated  by  the  solution  of  its  connective 
tissue.  The  fibers  thus  separated  are  readily  broken  up  into  particles 
by  solution  of  the  sarcolemma.  Well-cooked  meat  is  more  easily 
digested,  owing  to  the  conversion  of  the  connective  tissue  into 
gelatin. 

Connective  tissues  in  the  raw  or  imperfectly  gelatinized  condition 
are  very  slowly  dissolved.  Cartilage,  tendons,  and  even  bones  will 
in  time  be  corroded  and  liquefied. 

Vegetables  are  not  easily  digested  unless  thoroughly  prepared  by 
sufficient  cooking.  The  nutritive  principles  are  inclosed  by  cellulose 
walls,  which  are  not  affected  by  gastric  juice.  The  influence  of  heat 
and  moisture  softens  and  ruptures  the  cellulose  walls  so  as  to  permit 
the  introduction  of  gastric  juice  and  the  solution  of  its  nutritive 
principles. 

The  principal  action  of  the  gastric  juice,  however,  is  the  trans- 
formation of  the  different  proteid  principles  of  the  food  into  peptones, 
the  intermediate  stages  of  which  are  due  to  the  influence  of  the  acid 
and  pepsin  respectively.  As  soon  as  any  one  of  the  albumins  is  pene- 
trated by  the  acid  it  is  converted  into  acid-albumin,  a  fact  which 
indicates  that  the  first  step  in  gastric  digestion  is  the  acidification  of 
the  proteids.  This  having  been  accomplished,  the  pepsin  becomes 
operative  and  in  a  varying  length  of  time  transforms  the  acid-albu- 


DIGESTION. 


107 


min  into  a  new  form  of  proteid  termed  peptone.  In  this  transforma- 
tion it  is  possible  to  isolate  intermediate  bodies  by  the  addition  of 
ammonium  and  magnesium  sulphates,  to  which  the  term  albumoses 
or  proteases  has  been  given.  Because  of  the  order  in  which  they  are 
obtained  they  have  been  divided  into  primary  and  secondary.  The 
primary  proteoses  are  in  turn  separated  into  proto-  and  hetero- 
albumose ;  the  secondary  proteoses  are  termed  deutero-albumose. 
This  supposed  change  is  represented  by  the  following  scheme : 

Albumin. 

I 
Acid-albumin. 

C    Primary 
Albumoses 

(proto  and  hetero). 

Secondary 
Proteoses. 

•  I       (deutero). 

Peptone. 

Peptones. — Peptones  are  the  final  products  of  the  digestion  of 
proteid  bodies,  and  differ  from  the  bodies  from  which  they  are  de- 
rived in  the  following  particulars  : 

1.  They  are  diffusible, — i.  e.,  capable  of  passing  readily  through  ani- 
mal membranes, — a  condition  essential  for  their  absorption. 

2.  They  are  soluble  in  water  and  in  saline  solution. 

3.  They  are  non-coagulable  by  heat  and  nitric  or  acetic  acids.     They 
can    be    readily    precipitated,    however,    by    tannic    acid,    by    bile 
acids,  and  by  mercuric  chlorid. 

4.  They  are  absorbable  and  assimilable,  soon  becoming  transformed 
into  serum-albumin. 

The  duration  of  gastric  digestion  will  depend  largely  upon  the 
quantity  and  quality  of  the  food.  The  digestion  of  the  average  meal 
occupies  from  three  to  five  hours. 

Movements  of  the  Stomach. — During  the  period  of  gastric  diges- 
tion the  walls  of  the  stomach  become  the  seat  of  a  series  of  move- 
ments, somewhat  peristaltic  in  character,  which  serve  not  only  to 
incorporate  the  gastric  juice  with  the  food,  but  also  serve  to  eject 
the  liquefied  portions  of  the  food  into  the  intestine. 

After  the  entrance  of  the  food  both  the  cardiac  and  pyloric 
orifices  are  closed  by  the  contraction  of  their  sphincters.  Within 
five  minutes  (in  the  cat)  annular  constrictions  begin  in  the  pyloric 


108  HUMAN   PHYSIOLOGY. 

region  which  move  peristaltically  towards  the  pylorus.  As  diges- 
tion proceeds  these  constrictions  or  contractions  become  more  fre- 
quent and  more  vigorous.  The  result  is  a  trituration  and  liquefaction 
of  the  food.  So  soon  as  it  is  liquefied  the  pylorus  relaxes  and  per- 
mits of  its  discharge  into  the  intestine.  The  pylorus  then  closes 
and  further  preparation  of  food  goes  on.  From  time  to  time  the 
pylorus  relaxes  to  permit  the  discharge  of  prepared  and  liquefied 
food  until  digestion  is  completed.  In  the  cardiac  region  there  is  an 
absence  of  peristalsis  though  the  muscle  wall  is  in  a  state  of  active 
tone.  The  fundus  acts  as  a  reservoir  for  food  and  delivers  its  con- 
tents to  the  pyloric  region  as  rapidly  as  it  is  ready  to  receive  them. 

TABLE    SHOWING    DIGESTIBILITY    OF    VARIOUS    ARTICLES    OF    FOOD. 

Hours.  Minutes. 

Eggs,   whipped .     .     .     .   i  20 

"       soft-boiled 3 

4<       hard-boiled 3  30 

Oysters,  raw 2  55 

"         stewed 3  30 

Lamb,  broiled 2  30 

Veal,         "  4 

Pork,  roasted 5  15 

Beefsteak,    broiled 3 

Turkey,    roasted 2  25 

Chicken,  boiled 4 

"         fricasseed 2  45 

Duck,  roasted     . 4 

Soup,  barley,  boiled i  30 

"      bean,          "         3 

"      chicken,    '          3 

mutton,                3  30 

Liver,  beef,  broiled 2 

Sausage                            3  2° 

Green   corn,   boiled 3  45 

Beans,                  "          2  30 

Potatoes,   roasted 2  30 

"           boiled 3  3° 

Cabbage,          "          4  30 

Turnips,                     Z  3° 

Beets,               "          3  45 

Parsnips,         "          2  30 


DIGESTION.  109 

INTESTINAL    DIGESTION. 

The  process  of  digestion  as  it  takes  place  in  the  small  intestine  is 
exceedingly  important  and  complex,  and  is  brought  about  by  the 
action  of  the  pancreatic  juice,  the  bile,  and  the  intestinal  juice. 

The  contents  of  the  stomach  at  the  close  of  gastric  digestion  con- 
sist of  water,  inorganic  salts,  peptones,  undigested  albumins  and 
starches,  maltose,  cane-sugar,  liquefied  fats,  cellulose,  and  the  indi- 
gestible portions  of  meats,  cereals,  fruits,  etc.  This  so-called  chyme 
is  quite  acid  in  reaction,  and  upon  its  passage  through  the  now  open 
pylorus  into  the  intestine  it  excites  a  reflex  stimulation  and  secretion 
of  the  intestinal  fluids,  which  are  decidedly  alkaline  in  reaction,  and 
which  have  a  neutralizing  action  on  the  chyme.  As  soon  as  the  latter 
becomes  alkaline  and  gastric  digestion  is  arrested,  the  various  phases 
ot  intestinal  digestion  begin  which  eventuate  in  the  transformation 
of  all  the  remaining  undigested  nutritive  materials  into  absorbable 
and  assimilable  compounds. 

The  small  intestine  is  about  22  feet  in  length  and  about  i  H  inches 
in  diameter.  Like  the  stomach,  it  possesses  three  coats,  as  follows : 

1.  The  serous,  or  peritoneal. 

2.  The  muscular,  the  fibers  of  which  are  arranged  for  the  most  part 
circularly.     Some  of  the  fibers  are  so  arranged  as  to   form  longi- 
tudinal bands. 

3.  The  mucous,  which  presents  a  series  of  transverse  folds,  known  as 
the  valvula  conniventes. 

Intestinal  Glands. — In  that  portion  of  the  small  intestine  known 
as  the  duodenum  are  to  be  found  a  number  of  small,  branched,  tubu- 
lar glands  (Brunner's),  the  acini  of  which  are  lined  by  short, 
cylindric  cells,  similar  to  those  lining  the  pyloric  glands.  From  the 
duodenum  to  the  termination  of  the  intestine  the  mucous  membrane 
contains  an  enormous  number  of  tubular  glands  (Lieberkuhn's), 
formed  by  an  inversion  of  the  basement  membrane  and  lined  by 
epithelial  cells.  The  common  secretion  of  these  intestinal  glands 
forms  the  intestinal  juice.  This  is  a  thin,  opalescent,  slightly  yel- 
lowish fluid,  alkaline  in  reaction,  and  contains  water,  salts  and  pro- 
teid  matter. 

The  function  of  the  intestinal  juice  is  but  incompletely  known. 
It  appears  to  have  the  power  of  converting  starch  into  dextrose ; 
It  is  doubtful  whether  it  is  capable  of  digesting  either  albumins  or 


110 


HUMAN    PHYSIOLOGY. 


fats.  Its  most  distinctive  action  is  the  inversion  of  cane-sugar, 
maltose,  and  lactose  into  dextrose,  thus  preparing  them  for  absorp- 
tion. This  change  is  dependent  on  the  presence  of  a  ferment  body 
known  as  invertin. 

The  pancreatic  juice  is  secreted  by  the  pancreas,  a  flattened  gland, 
about  six  inches  long,  running  transversely  across  the  posterior  wall 
of  the  abdomen  behind  the  stomach ;  its  duct  opens  into  the  duo- 
denum. 

The  pancreas  is  similar  in  structure  to  the  salivary  glands,  and 
consists  of  a  system  of  ducts  terminating  in  acini.  The  acini  are 
tubular  or  flask-shaped,  and  consist  of  a  basement  membrane  lined 
by  a  layer  of  cylindric,  conic  cells,  which  encroach  upon  the  lumen 
of  the  acini.  The  cells  exhibit  a  difference  in  their  structure  (Fig. 
14),  and  may  be  said  to  consist  of  two  zones — viz.,  an  outer  parietal 


FIG.  14. — ONE  SACCULE  OF  THE  PANCREAS  OF  THE  RABBIT  IN  DIFFERENT 
STATES  OF  ACTIVITY. — (Yeo's  "Text-book  of  Physiology,"  after  Kuhne 
and  Lea.) 

A.  After  a  period  of  rest,  in  which  case  the  outlines  of  the  cells  are  indistinct 
and  the  inner  zone — i.  e.,  the  part  of  the  cells  (a)  next  the  lumen  (c)— is 
broad  and  filled  with  fine  granules.  B.  After  the  gland  has  poured  out  its 
secretion,  when  the  cell  outlined  (d)  are  clearer,  the  granular  zone  (a)  is 
smaller,  and  the  clear  outer  zone  is  wider. 

zone,  which  is  transparent  and  apparently  homogeneous,  staining 
rapidly  with  carmin  ;  an  inner  zone,  which  borders  the  lumen,  and 
is  distinctly  granular  and  stains  but  slightly  with  carmin.  These  cells 
undergo  changes  similar  to  those  exhibited  by  the  cells  of  the  salivary 
glands  during  and  after  active  secretion.  As  soon  as  the  secretory 
activity  of  the  pancreas  is  established,  the  granules  disappear,  and  the 
inner  granular  layer  becomes  reduced  to  a  very  narrow  border, 


DIGESTION.  Ill 

while  the  outer  zone  increases  in  size  and  occupies  nearly  the  entire 
cell.  During  the  intervals  of  secretion,  however,  the  granular  layer 
reappears  and  increases  in  size  until  the  outer  zone  is  reduced  to  a 
minimum.  It  would  seem  that  the  granular  matter  is  formed  by 
the  nutritive  processes  occurring  in  the  gland  during  rest,  and  is 
discharged  during  secretory  activity  into  the  ducts,  and  takes  part  in 
the  formation  of  the  pancreatic  secretion. 

The  pancreatic  juice  is  transparent,  colorless,  strongly  alkaline, 
and  viscid,  and  has  a  specific  gravity  of  1040.  It  is  one  of  the  most 
important  of  the  digestive  fluids,  as  it  exerts  a  transforming  influence 
upon  all  classes  of  alimentary  principles,  and  has  been  shown  to  con- 
tain at  least  three  distinct  ferments.  It  has  the  following  com- 
position : 

COMPOSITION    OF    PANCREATIC    JUICE. 

Water 900.76 

Albuminoid  substances ...        90.44 

Inorganic  salts 8.80 


1,000.00 

The  pancreatic  juice  is  characterized  by  its  action : 

1.  Upon  starch.     When  starch  is  subjected  to  the  action  of  the  juice, 
it  is  at  once  transformed  into  maltose ;  the  change  takes  place  more 
rapidly  than  when  saliva  is  added.     This  action  is  caused  by  the 
presence  of  a  special  ferment,  amylopsin. 

2.  Upon    albumin.      The    proteid    bodies    which    escape    digestion    in 
the  stomach  are  converted  into  peptones  by  the  action  of  the  alkali 
and  ferment.     The  first  effect  of  the  alkali  is  to  change  the  proteid 
into  an  alkali-albumin,  a  fact  which  indicates  that  in  the  digestion 
of   albumin   by   pancreatic    juice,    the   first   stage   is    alkalinization. 
This  having  been  accomplished,  the  ferment  trypsin  transforms  the 
alkali-albumin  into  peptone.     For  the  same  reasons  it  is  believed 
that  here  also  these  bodies   are  preceded  in  their  development  by 
albumoses,  of  which  there  are  probably  two   forms.     Long-contin- 
ued action  of  the  pancreatic  juice,  as  previously  stated,  decomposes 
the  peptone  into  leucin,  tyrosin,  etc. 

3.  Upon  fats.     The  most  striking  action  of  the  pancreatic  juice  is  the 
emulsification  of  the  fats  or  their  subdivision  into  minute  particles 
of  microscopic  size.     This  change  takes  place  rapidly,  and  depends 
upon  the  alkalinity  of  the  fluid  and  the  quantity  of  albumin  present, 


112  HUMAN   PHYSIOLOGY. 

combined  with  the  intestinal  movements.  The  neutral  fats  are  also 
decomposed  into  their  corresponding  fatty  acids  and  glycerin; 
The  acids  thus  set  free  unite  with  the  alkaline  bases  present,  in  the 
intestine  and  form  soaps.  This  decomposition  of  the  neutral  fats 
is  caused  by  the  ferment,  steapsin. 

The  bile  has  an  important  function  in  the  elaboration  of  the  food 
and  in  its  preparation  for  absorption.  It  is  a  golden-brown,  viscid 
fluid,  having  a  neutral  or  alkaline  reaction  and  a  specific  gravity 
of  1020. 

COMPOSITION    OF    BILE. 

Water 859.2 

Sodium  glycocholate  -\ 
Sodium  taurocholate  [ 

Fat 9.2 

Cholesterin        2.6 

Mucus  and  coloring-matter 29.8 

Salts 7-8 


1,000.0 

The  biliary  salts,  sodium  glycocholate  and  taurocholate,  are  char- 
acteristic ingredients,  and  by  the  process  of  secretion  are  formed  in 
the  liver  from  materials  furnished  by  the  blood.  It.  is  probable 
that  they  are  derived  from  the  nitrogenized  compounds,  though  the 
stages  in  the  process  are  unknown.  They  are  reabsorbed  from  the 
small  intestine  to  play  some  ulterior  part  in  nutrition. 

Cholesterin  is  a  product  of  waste  taken  up  by  the  blood  from  the 
nerve  tissues  and  excreted  by  the  liver.  It  crystallizes  in  the  form 
of  rhombic  plates  which  are  quite  transparent.  When  retained 
within  the  blood,  it  gives  rise  to  the  condition  of  cholesteremia, 
attended  with  severe  nervous  symptoms.''  It  is  given  off  in  the 
feces  under  the  form  of  stercorin. 

The  coloring-matters  which  give  the  tints  to  the  bile  are  biliverdin 
and  bilirubin,  and  are  probably  derived  from  the  coloring-matter  of 
the  blood.  Their  presence  in  any  fluid  can  be  recognized  by  adding 
to  it  nitric  acid  containing  nitrous  acid,  when  a  play  of  colors  is 
observed,  beginning  with  green,  blue,  violet,  red  and  yellow. 

The  bile  is  both  a  secretion  and  an  excretion ;  it  is  constantly 
being  formed  and  discharged  by  the  hepatic  ducts  into  the  gall- 


DIGESTION.  113 

bladder,  in  which  it  is  stored  up  during  the  intervals  of  digestion. 
As  soon  as  food  enters  the  intestines  it  is  poured  out  abundantly  by 
the  contraction  of  the  walls  of  the  gall-bladder. 

The  amount  secreted  in  twenty-four  hours  is  about  2l/2  pounds. 

Functions  of  the  Bile: 

1.  It   assists    in   the    emulsification    of   the    fats    and   promotes    their 
absorption. 

2.  It  tends  to  prevent  putrefactive  changes  in  the  food. 

3.  It   stimulates   the   secretion   of   the   intestinal   glands,    and   excites 
the  normal  peristaltic  movement  of  the  bowels. 

The  digested  food,  the  chyme,  is  a  grayish,  pultaceous  mass,  but 
as  it  passes  through  the  intestines  it  becomes  yellow  from  admixture 
with  the  bile.  It  is  propelled  onward  by  vermicular  motion — by  the 
contraction  of  the  circular  and  longitudinal  muscle-fibers. 

During  the  passage  of  the  digesting  food  through  the  intestinal 
canal  the  nutritive  products — the  peptones,  the  dextrose  and  levulose, 
the  fatty  emulsions,  the  fatty  acids  and  their  soaps — are  absorbed 
into  the  blood,  while  the  undigested  residue  is  carried  onward  by  the 
peristaltic  movements  through  the  ileo-cecal  valve  into  the  large 
intestine. 

Intestinal  Fermentation. — Owing  to  the  favorable  conditions  for 
fermentative  and  putrefactive  processes — e.  g.,  heat,  moisture,  oxygen, 
microorganisms — the  food,  when  consumed  in  excessive  quantity  or 
when  acted  upon  by  defective  secretions,  undergoes  a  series  of  de- 
composition changes  which  are  attended  by  the  production  of  gases 
and  various  chemic  compounds.  Grape-sugar  and  maltose  are  par- 
tially split  into  lactic  acid,  this  into  butyric  acid,  carbon  dioxid, 
and  hydrogen.  Fats  are  reduced  to  glycerol  and  fatty  acids ;  the 
glycerol,  according  to  the  organisms  present,  yields  succinic  and 
other  fatty  acids,  carbon  dioxid,  and  hydrogen. 

The  proteids,  under  the  prolonged  action  of  the  pancreatic  juice, 
are  decomposed,  and  yield  leucin  and  tyrosin  ;  the  former  is  split  into 
valerianic  acid,  ammonia,  and  carbon  dioxid ;  the  latter  is  split 
into  indol,  which  is  the  antecedent  of  indican  in  the  urine.  Skatol  is 
another  proteid  derivative  constantly  present  in  the  fecal  substance. 

Intestinal    Movements. — During    intestinal    digestion    the    walls 
of   the   intestine   exhibit   two.  kinds    of   movement,    viz.,    a    rhythmic 
segmentation  and  a  peristalsis.      By  the   former  the   food  is   divided 
9 


114  HUMAN   PHYSIOLOGY. 

into  segments  and  by  the  latter,  it  is  carried  down  the  intestine. 
Shortly  after  the  entrance  of  the  food  into  the  intestine,  segmenta- 
tion begins  by  a  contraction  of  bands  of  circular  muscle  fibers.  So 
soon  as  a  mass  of  food  is  divided  into  segments  each  segment  is  in 
turn  again  divided  by  similar  contractions.  The  lower  half  of  each 
segment  then  unites  with  the  upper  half  of  the  segment  below  to 
commingle  with  it  and  to  expose  new  surfaces  of  the  food  mass 
to  contact  with  the  intestinal  juices  and  to  the  mucous  membrane. 
A  continual  repetition  of  this  process  results  in  a  thorough  mixing 
of  the  food  with  the  digestive  juices.  Subsequent  peristaltic  waves 
slowly  carry  the  food  down  the  intestine. 

The  large  intestine  extends  from  the  ileo-cecal  valve  to  the  anus, 
and  is  about  five  feet  in  length.  Like  the  stomach  it  consists  of  three 
coats :  the  serous,  the  muscular,  and  mucous.  The  mucous  membrane 
contains  a  number  of  mucous  glands,  the  secretion  from  which 
lubricates  the  surface  of  the  canal.  The  ascending  portion  of  the 
large  intestine  possesses  the  power  of  absorption,  and  hence  its 
contents  become  less  liquid  and  more  consistent.  As  the  residue 
passes  toward  the  sigmoid  flexure  it  acquires  the  characteristics  of 
fecal  matter.  This  residue  consists  of  the  undigested  portions  of 
the  food,  decomposition  products,  mucus,  and  inorganic  salts. 

Defecation  is  the  voluntary  act  of  extruding  the  feces  from  the 
rectum,  and  is  accomplished  by  a  relaxation  of  the  sphincter  ani 
muscle  and  by  the  contraction  of  the  muscular  walls  of  the  rectum, 
aided  by  the  contraction  of  the  abdominal  muscles. 


ABSORPTION. 

The  term  absorption  is  applied  to  the  passage  or  transference  of 
material  into  the  blood  from  the  tissues,  from  the  serous  cavities, 
and  from  the  mucous  surfaces  of  the  body.  The  most  important  of 
these  surfaces,  especially  in  its  relation  to  the  formation  of  blood, 
is  the  mucous  surface  of  the  alimentary  canal ;  for  it  is  from  this 
organ  that  new  materials  are  derived  which  maintain  the  quality 
and  quantity  of  the  blood.  The  absorption  of  materials  from  the 
interstices  of  the  tissues  is  to  be  regarded  rather  as  a  return  to  the 
blood  of  liquid  nutritive  material  which  has  escaped  from  the 
blood-vessels  for  nutritive  purposes,  and  which,  if  not  returned,  would 


ABSORPTION.  115 

lead  to  an  accumulation  of  such  fluid  and  the  development  of  drop- 
sical  conditions. 

The  anatomic  mechanisms  involved  in  the  absorptive  processes  are, 
primarily,  the  lymph-spaces,  the  lymph-capillaries,  and  the  blood- 
capillaries  ;  secondarily,  the  lymphatic  vessels  and  larger  blood-vessels. 

Lymph-spaces,  Lymph-capillaries,  Blood-capillaries. — Everywhere 
throughout  the  body,  in  the  intervals  between  connective-tissue  bun- 
dles and  in  the  interstices  of  the  several  structures  of  which  an  organ 
is  composed,  are  found  spaces  of  irregular  shape  and  size,  determined 
largely  by  the  nature  of  the  organ  in  which  they  are  found,  which 
have  been  termed  lymph-spaces  or  lacuna,  from  the  fact  that  during 
the  living  condition  they  are  continually  receiving  the  lymph  which 
has  escaped  from  the  blood-vessels  throughout  the  body.  In  addi- 
tion to  the  connective-tissue  lymph-spaces,  various  observers  have 
described  special  lymph-spaces  in  the  testicle,  kidney,  liver,  thymus 
gland,  and  spleen ;  in  all  secreting  glands  between  the  basement 
membrane  and  blood-vessels ;  around  blood-vessels  (perivascular 
spaces),  and  around  nerves.  The  serous  cavities  of  the  body — peri- 
toneal, pleural,  pericardial,  etc. — may  also  be  regarded  as  lymph- 
spaces,  which  are  in  direct  communication  by  open  mouths  or 
stomata  with  the  lymph  capillaries.  This  method  of  communica- 
tion is  not  only  true  of  serous  membranes,  but  to  some  extent  also 
of  mucous  membranes.  The  cylindric  sheaths  and  endothelial  cells 
surrounding  the  brain,  spinal  cord,  and  nerves  can  also  be  looked 
upon  as  lymph-spaces  in  connection  with  lymph-capillaries. 

The  lymph-capillaries,  in  which  the  lymph-vessels  proper  take 
their  origin,  are  arranged  in  the  form  of  plexuses  of  quite  irregu- 
lar shape.  In  most  situations  they  are  intimately  interwoven  with 
the  blood-vessels,  from  which,  however,  they  can  be  readily  dis- 
tinguished by  their  larger  caliber  and  irregular  expansions.  The 
wall  of  the  lymph-capillary  is  formed  by  a  single  layer  of  epithelioid 
cells,  with  sinuous  outlines,  and  which  accurately  dovetail  with  one 
another.  In  no  instance  are  valves  found.  In  the  villus  of  the  small 
intestine  the  beginning  of  the  lymphatic  is  to  be  regarded  as  a  lymph- 
capillary,  generally  club-shaped,  which  at  the  base  of  the  villus  enters 
a  true  lymphatic ;  at  this  point  a  valve  is  situated,  which  prevents 
regurgitation.  The  lymph  capillaries  anastomose  freely  with  one 
another,  and  communicate  on  the  one  hand  with  the  lymph-spaces, 
and  on  the  other  with  the  lymphatic  vessels  proper, 


116  HUMAN    PHYSIOLOGY. 

As  the  shape,  size,  etc.,  of  both  lymph-spaces  and  capillaries  are 
determined  largely  by  the  nature  of  the  tissues  in  which  they  are 
contained,  it  is  not  always  possible  to  separate  the  one  from  the  other. 
Their  function,  however,  may  be  regarded  as  similar — viz.,  the  col- 
lection of  the  lymph  which  has  escaped  from  the  blood-vessels,  and 
its  transmission  onward  into  the  regular  lymphatic  vessels. 

The  blood-capillaries  not  only  permit  the  escape  of  the  liquid 
nutritive  portions  of  the  blood  through  their  delicate  walls,  but  are 
also  engaged  in  the  reabsorption  of  this  transudate,  as  well  as  in  the 
absorption  of  new  materials  from  the  alimentary  canal.  The  exten- 
sive capillary  network  which  is  formed  by  the  ultimate  subdivision 
of  the  arterioles  in  the  submucous  tissue  and  villi  of  the  small  intes- 
tine forms  an  anatomic  arrangement  well  adapted  for  absorption.  It 
is  now  well  known  that  in  the  absorption  of  the  products  of  digestion 
the  blood-capillaries  are  more  active  than  the  lymph-capillaries. 

Lymph-Vessels. — These  constitute  a  system  of  minute,  delicate 
transparent  vessels,  found  in  nearly  all  the  organs  and  tissues  of  the 
the  body.  Having  their  origin  at  the  periphery  in  the  lymph-capil- 
laries and  spaces,  they  rapidly  converge  toward  the  trunk  of  the 
body  and  empty  into  the  thoracic  duct.  In  their  course  they  pass 
through  numerous  small  ovoid  bodies,  the  lymphatic  glands. 

The  lymph-vessels  of  the  small  intestines — the  lacteals — arise 
within  the  villous  processes  which  project  from  the  inner  surface  of 
the  intestine  throughout  its  entire  extent.  The  wall  of  the  villus 
is  formed  by  an  elevation  of  the  basement  membrane,  and  is  cov- 
ered by  a  layer  of  columnar  epithelial  cells.  The  basis  of  the  villus 
consists  of  adenoid  tissue,  a  fine  plexus  of  blood-vessels,  unstriped 
muscle-fibers,  and  the  lacteal  vessel.  The  adenoid  tissue  consists 
of  a  number  of  intercommunicating  spaces,  containing  leukocytes. 
The  lacteal  vessel  possesses  a  thin  but  distinct  wall  composed  of  en- 
dothelial  plates,  with  here  and  there  openings  which  bring  the 
interior  of  the  villus  into  communication  with  the  spaces  of  the 
adenoid  tissue. 

The  structure  of  the  larger  vessels  resembles  that  of  the  veins, 
consisting  of  three  coats  : 

1.  External,  composed  of  fibrous  tissue  and  muscle  fibers,   arranged 
longitudinally. 

2.  Middle,  consisting  of  white  fibers  and  yellow  elastic  tissue,  non- 
striated  muscle-fibers,  arranged  transversely. 


ABSORPTION.  1 17 

3.  Internal,  composed  of   an   elastic   membrane,   lined   by   endothelial 

cells. 

Throughout  their  course  are  found  numerous  semilunar  valves, 
opening  toward  the  larger  vessels,  formed  by  a  folding  of  the  inner 
coat  and  strengthened  by  connective  tissue. 

Lymph  Glands. — The  lymph  glands  consist  of  an  external  cap- 
sule composed  of  fibrous  tissue  which  contains  non-striped  muscle- 
fibers  ;  from  its  inner  surface  septa  of  fibrous  tissue  pass  inward 
and  subdivide  the  gland-substance  into  a  series  of  compartments, 
which  communicate  with  one  another.  The  blood-vessels  which  pene- 
trate the  gland  are  surrounded  by  fine  threads,  forming  a  follicular 
arrangement,  the  meshes  of  which  contain  numerous  lymph-cor- 
puscles. Between  the  follicular  threads  and  the  wall  of  the  gland 
lies  a  lymph-channel  traversed  by  a  reticulum  of  adenoid  tissue. 
The  lymph-vessels,  after  penetrating  this  capsule,  pour  their  lymph 
into  this  channel,  through  which  it  passes ;  it  is  then  collected 
by  the  efferent  vessels  and  transmitted  onward.  The  lymph-cor- 
puscles which  are  washed  out  of  the  gland  into  the  lymph-stream  are 
formed,  most  probably,  by  division  of  preexisting  cells. 

The  thoracic  duct  is  the  general  trunk  of  the  lymphatic  system ; 
into  it  the  vessels  of  the  lower  extremities,  of  the  abdominal  organs, 
of  the  left  side  of  the  head,  and  of  the  left  arm  empty  their  contents. 
It  is  about  twenty  inches  in  length,  arises  in  the  abdomen,  opposite 
the  third  lumbar  vertebra,  by  a  dilatation  (the  receptaculum  chyli}, 
ascends  along  the  vertebral  column  to  the  seventh  cervical  vertebra, 
and  terminates  in  the  venous  system  at  the  junction  of  the  internal 
jugular  and  subclavian  veins  on  the  left  side.  The  lymphatics  of  the 
right  side  of  the  head,  of  the  right  arm,  and  of  the  right  side  of  the 
thorax  terminate  in  the  right  thoracic  duct,  about  one  inch  in 
length,  which  joins  the  venous  system  at  the  junction  of  the  internal 
jugular  and  subclavian  on  the  right  side. 

The  general  arrangement  of  the  lymph  vessels  is  shown  in  figure  15. 

The  blood-vessels  which  are  concerned  in  the  conduction  of  fresh 
nutritive  material  from  the  alimentary  canal  have  their  origin  in 
the  elaborate  capillary  network  in  the  mucous  membrane.  The  small 
veins  which  emerge  from  the  network  gradually  unite,  forming 
larger  and  larger  trunks,  which  are  known  as  the  gastric,  superior, 
and  inferior  mesenteric  veins.  These  finally  unite  to  form  the  portal 
vein,  a  short  trunk  about  three  inches  in  length.  The  portal  vein 


118 


HUMAN   PHYSIOLOGY. 


FIG.    15. — DIAGRAM    SHOWING   THE    COURSE   OF   THE   MAIN    TRUNKS   OF   THE 
ABSORBENT    SYSTEM. — (Yeo's    "Text-book    of  Physiology.") 

The  lymphatics  of  lower  extremities  (D)  meet  the  lacteals  of  intestines  (LAC) 
at  the  receptaculum  chyli  (RC),  where  the  thoracic  duct  begins.  The 
superficial  vessels  are  shown  in  the  diagram  on  the  right  arm  and  leg  (S), 
and  the  deeper  ones  on  the  arm  to  the  left  (D).  The  glands  are  here  and 
there  shown  in  groups.  The  small  right  duct  opens  into  the  veins  on  the 
right  side.  The  thoracic  duct  opens  into  the  union  of  the  great  veins  of 
the  left  side  of  the  neck  (T). 


ABSORPTION. 


119 


enters  the  liver  at  the  transverse  fissure,  after  which  it  forms  a 
fine  capillary  plexus  ramifying  throughout  the  substance  of  the 
liver ;  from  this  plexus  the  hepatic  veins  take  their  origin,  and 
finally  empty  the  blood  into  the  vena  cava  inferior.  (See  Fig.  16.) 


FIG.    1 6. 

Diagram  of  the  portal  vein  (pv)  arising  in  the  alimentary  tract  and  spleen  (s), 
and  carrying  the  blood  from  these  organs  to  the  liver. —  (Yeo's  "Text- 
book of  Physiology.") 

Absorption  of  Food. — Physiological  experiments  have  demonstrated 
that  the  agents  concerned  in  the  absorption  of  new  materials  from 
the  alimentary  canal  are  : 

1.  The  blood-vessels  of  the  entire  canal,  but  more  particularly  those 
uniting  to  form  the  portal  vein. 

2.  The  lymph  vessels  coming  from  the  small  intestine,  which  converge 
to  empty  into  the  thoracic   duct. 

As  a  result  of  the  action  of  the  digestive  fluids  upon  the  different 


120 


HUMAN    PHYSIOLOGY. 


classes  of  food  principles — albumins,  sugars,  starches,  and  fats — 
there  are  formed  peptones,  glucose,  and  'fatty  emulsion,  which  differ 
from  the  former  in  being  highly  diffusible — a  condition  essential  to 
their  absorption.  In  order  that  these  substances  may  get  into  the 
blood,  they  must  pass  through  the  layer  of  cylindric  epithelial  cells 
and  the  underlying  basement  membrane,  and  into  the  lymph-spaces 
of  the  villi  and  submucous  tissue.  The  mechanism  by  which  the  cells 
effect  this  passage  of  the  food  is  but  imperfectly  understood.  Os- 
mosis and  filtration  are  conditions,  however,  made  use  of  by  the 
cells  in  the  absorptive  process. 

The  products  of  digestion  find  their  way  into  the  general  circula- 
tion by  two  routes  : 

1.  The  water,  peptones,  glucose,  and  soluble  salts,  after  passing  into 
the  lymph-spaces  of  the  villi,  pass  through  the  wall  of  the  capillary 
blood-vessel ;    entering    the    blood,    they    are    carried    to    the    liver 
by  the  vessels  uniting  to  form  the  portal  vein  ;  emerging  from  the 
liver,  they  are  emptied  into  the  inferior  vena  cava  by  the  hepatic 
vein. 

2.  The   emulsified   fat   enters   the   lymph-capillary   in   the   interior   of 
the   villus ;   by   the   contraction   of  the   layer   of   muscle-fibers   sur- 
rounding it  its  contents  are  forced  onward  into  the  lymph-vessels 
or  lacteals,  thence  into  the  thoracic  duct,  and  finally  into  the  blood 
stream    at    the    junction    of    the    internal    jugular    and    subclavian 
veins  on  the  left  side. 

Properties  and  Composition  of  Lymph. — Lymph,  as  found  in  the 
lymph-vessels  of  animals,  is  a  clear,  colorless,  or  opalescent  fluid, 
having  an  alkaline  reaction,  a  saline  taste,  and  a  specific  gravity  of 
about  1040.  It  holds  in  suspension  a  number  of  corpuscles  resem- 

COMPOSITION    OF    LYMPH. 

Water 95-536 

Proteids  (serum-albumin,  fibrin-globulin)  .     .     .  1.320 

Extractives   (urea,  sugar,  cholesterin)   .     .     .     .  1-559 

Fatty  matters a  trace 

Salts        0.585 


bling  in  their  general  appearance  the  white  corpuscles  of  the  blood. 
Their    number    has    been    estimated    at    8,200    per    cubic    millimeter, 


ABSORPTION.  121 

though  the  number  varies  in  different  portions  of  the  lymphatic 
system.  As  the  lymph  flows  through  the  lymphatic  gland  it  receives 
a  large  addition  of  corpuscles.  Lymph-corpuscles  are  granular  in 
structure,  and  measure  •%•£$•$  °^  an  incn  in  diameter.  When  with- 
drawn from  the  vessels,  lymph  undergoes  a  spontaneous  coagulation 
similar  to  that  of  blood,  after  which  it  separates  in  serum  and  clot. 

Origin  and  Functions  of  Lymph. — Though  the  blood  is  the  com- 
mon reservoir  of  all  nutritive  materials,  they  are  not  available  for 
nutritive  purposes  as  long  as  they  are  confined  within  the  blood- 
vessels. But  owing  to  the  character  of  the  wall  of  the  capillary 
blood-vessel,  some  of  the  constituents  of  the  blood-plasma  pass 
through  it  and  are  received  by  the  tissue-spaces  in  which  they  come 
into  contact  with  the  tissue-cells.  To  the  sum  total  of  these  materials 
the  term  lymph  is  given.  Its  function  becomes  apparent  from  its 
origin  and  composition,  its  situation  and  relation  to  the  tissues.  It 
is  to  furnish  the  tissue-cells  with  those  nutritive  materials  which 
are  necessary  for  their  growth,  repair  and  functional  activity.  It 
also  receives  all  waste  products  that  arise  from  their  activity  prior 
to  their  removal  by  the  blood-  and  lymph-vessels. 

Absorption  of  Lymph. — From  the  fact  that  lymph  is  being  dis- 
charged more  or  less  continuously  from  the  thoracic  duct,  it  is 
evident  that  lymph  is  being  absorbed  from  the  intercellular  spaces ; 
from  which  it  may  be  inferred  that  more  lymph  is  passing  from 
the  blood  into  the  tissue-spaces  than  is  necessary  for  the  immediate 
needs  of  the  tissues.  To  prevent  an  accumulation  and  an  inter- 
ference through  pressure  with  the  activities  of  the  tissues,  the  excess 
is  absorbed  by  the  lymph-vessels  and  returned  to  the  blood  stream 
by  way  of  the  thoracic  duct.  It  is  likely  that  some  of  the  con- 
stituents are  absorbed  by  the  blood-vessels. 

Chyle. — Chyle  is  the  fluid  found  in  the  lymph  vessels,  coming 
from  the  small  intestine  after  the  digestion  of  a  -neal  containing 
fat.  In  the  intervals  of  digestion  the  fluid  of  thebe  lymphatics  is 
identical  in  all  respects  with  the  lymph  found  in  all  other  regions 
of  the  body.  As  soon  as  the  emulsified  fat  passes  into  the  lymph 
vessels  and  mingles  with  the  lymph  it  becomes  milky  white  in  color, 
and  the  vessels  which  previously  were  invisible  become  visible,  and 
resemble  white  threads  running  between  the  layers  of  the  mesentery. 
Chyle  has  a  composition  similar  to  that  of  lymph,  but  it  contains, 


122  HUMAN   PHYSIOLOGY. 

in  addition,  numerous  fatty  granules,  each  surrounded  by  an  albu- 
minous envelope.  When  examined  microscopically,  the  chyle  pre- 
sents a  fine  molecular  basis,  made  up  of  the  finely  divided  granules 
of  fat. 

COMPOSITION    OF    CHYLE. 

Water 902.37 

Albumin 35-i6 

Fibrinogen 3.70 

Extractives 15.65 

Fatty   matters 36.01 

Salts 7.11 


Forces  Aiding  the  Movement  of  Lymph  and  Chyle. — The  lymph 
and  chyle  are  continually  moving  in  a  progressive  manner  from  the 
.  periphery  or  beginning  of  the  lymphatic  system  to  the  final  termina- 
tion of  the  thoracic  duct.  The  force  which  primarily  determines 
the  movement  of  the  lymph  has  its  origin  in  the  beginnings  of  the 
lymph-vessels,  and  depends  upon  the  difference  in  pressure  here 
and  the  pressure  in  the  thoracic  duct.  The  greater  the  quantity  of 
fluid  poured  into  the  lymph-spaces,  the  greater  will  be  the  pressure 
and,  consequently,  the  movement.  The  first  movement  of  chyle  is 
the  result  of  a  contraction  of  the  muscle-fibers  within  the  walls  of 
the  villus.  At  the  time  of  contraction  the  lymph  capillary  is  com- 
pressed and  shortened,  and  its  contents  are  forced  onward  into 
the  true  lymphatic.  When  the  muscle-fibers  relax,  regurgitation  is 
prevented  by  the  closure  of  the  valve  in  the  lymphatic  at  the  base  of 
the  villus. 

As  the  walls  of  the  lymph  vessels  contain  muscle-fibers,  when 
they  become  distended  these  fibers  contract  and  assist  materially  in 
the  onward  movement  of  the  fluid. 

The  contraction  of  the  general  muscular  masses  in  all  parts  of  the 
body,  by  exerting  an  intermittent  pressure  upon  the  lymphatics,  also 
hastens  the  current  onward  ;  regurgitation  is  prevented  by  the  closure 
of  valves  which  everywhere  line  the  interior  of  the  vessels. 

The  respiratory  movements  aid  the  general  flow  of  both  lymph 
and  chyle  from  the  thoracic  duct  into  the  venous  blood.  During 
the  time  of  an  inspiratory  movement  the  pressure  within  the  thorax, 
but  outside  the  lungs,  undergoes  a  diminution  in  proportion  to  the 


BLOOD.  123 

extent  of  the  movement ;  as  a  result,  the  fluid  in  the  thoracic  duct 
outside  of  the  thorax,  being  under  a  higher  pressure,  flows  more 
rapidly  into  the  venous  system.  At  the  time  of  an  expiration,  the 
pressure  rises  and  the  flow  is  temporarily  impeded,  only  to  begin 
again  at  the  next  inspiration. 


BLOOD. 

The  blood  is  a  nutritive  fluid  containing  all  the  elements  necessary 
for  the  repair  of  the  tissues ;  it  also  contains  principles  of  waste 
absorbed  from  the  tissues,  which  are  conveyed  to  the  various  excre- 
tory organs  and  by  them  eliminated  from  the  body. 

The  total  amount  of  blood  in  the  body  is  estimated  to  be  about 
one  thirteenth  of  the  body-weight ;  from  ten  to  twelve  pounds  in  an 
individual  of  average  physical  development.  The  quantity  varies 
during  the  twenty-four  hours,  the  maximum  being  reached  in  the 
afternoon,  the  minimum  in  the  early  morning  hours. 

Blood  is  an  opaque,  red  fluid,  having  an  alkaline  reaction,  a  saline 
taste,  and  a  specific  gravity  of  1055. 

The  opacity  is  due  to  the  refraction  of  the  rays  of  light  by  the 
elements  of  which  the  blood  is  composed.  The  color  varies  in  hue, 
from  a  bright  scarlet  in  the  arteries  to  a  deep  purple  in  the  veins, 
due  to  the  presence  of  a  coloring-matter — hemoglobin — in  different 
degrees  of  oxidation. 

The  alkalinity  is  constant,  and  depends  upon  the  presence  of  the 
alkaline  sodium  phosphate,  Na2HPO4. 

The  saline  taste  is  due  to  the  amount  of  sodium  chlorid  present. 

The  specific  gravity,  within  the  limits  of  health,  ranges  from  1045 
to  1075. 

The  odor  of  the  blood  is  characteristic,  and  varies  with  the  animal 
from  which  it  is  drawn  ;  it  is  due  to  the  presence  of  caproic  acid. 

The  temperature  of  the  blood  ranges  from  98°  F.  at  the  surface 
to  107°  F.  in  the  hepatic  vein;  it  loses  heat  by  radiation  and  evapora- 
tion as  it  approaches  the  extremities  and  as  it  passes  through  the 
lungs. 

Blood  Consists  of  Two  Portions: 

i.  The  liquor  sanguinis  or  plasma,  a  transparent,   colorless  fluid,  in 
which  are  floating — 


124  HUMAN   PHYSIOLOGY. 

the  joints  where  the  movement  takes  place,  and  when  the  muscles 
are  considered  as  sources  of  power  for  imparting  movement  to  the 
levers,  with  the  object  of  overcoming  resistance  or  raising  weights. 

In  mechanics,  levers  of  three  kinds  or  orders  are  recognized,  ac- 
cording to  the  relative  position  of  the  fulcrum  or  axis  of  motion,  the 
applied  power,  and  the  weight  to  be  moved.     (See  Fig.  5.) 
2.  Red  and  white  corpuscles,  these  constituting  by  weight  less  than 

one  half  (40  per  cent.)  of  the  entire  amount  of  blood. 

COMPOSITION    OF    PLASMA. 

Dalton, 

Water 902.00 

Albumin 53*oo 

Paraglobulin         22.00 

Fibrinogen 3.00 

Fatty   matters 2.50 

Crystallizable  nitrogenous   matters 4.00 

Other  organic  matter  . 5.00 

Mineral  salts 8.50 


1,000.00 

Water  acts  as  a  solvent  for  the  inorganic  matters  and  holds  in 
suspension  the  corpuscular  elements. 

Albumin  is  the  nutritive  principle  of  the  blood  ;  it  is  absorbed  by 
the  tissues  to  repair  their  waste  and  is  transformed  into  the  organic 
basis  characteristic  of  each  structure. 

Paraglobulin  or  fibrinoplastin  is  a  soft,  amorphous  substance  pre- 
cipitated by  sodium  chlorid  in  excess,  or  by  passing  a  stream  of  car- 
bonic acid  through  dilute  serum. 

Fibrinogen  also  can  be  obtained  by  strongly  diluting  the  serum  and 
passing  carbonic  acid  through  it  for  a  long  time,  when  it  is  precipi- 
tated as  a  viscous  deposit. 

Fatty  matter  exists  in  the  blood  to  the  extent  of  about  0.25  per 
cent.  Just  after  a  meal  rich  in  fat,  this  amount  may  be  considerably 
increased.  Within  a  few  hours  it  disappears,  though  its  ultimate 
fate  is  unknown. 

Sugar  is  represented  by  dextrose.  The  amount  present  varies 
from  o.i  to  0.3  per  cent.  It  is  derived  directly  from  the  glycogen  of 
the  liver.  Should  the  normal  percentage  be  increased,  the  sugar 
is  eliminated  by  the  kidneys. 


BLOOD.  125 

The  inorganic  constituents  are  chiefly  sodium  and  potassium  chlo- 
rids,  sulphates  and  phosphates  together  with  calcium  and  mag- 
nesium  phosphates.  The  sodium  chlorid  is  the  most  abundant, 
amounting  to  about  5.5  parts  per  thousand.  The  alkaline  salts 
impart  the  alkaline  reaction  and  promote  the  absorption  from  the 
tissues  of  the  carbon  dioxid. 

Excrementitious  matters  are  represented  by  carbonic  acid,  urea, 
creatin,  creatinin,  urates,  oxalates,  etc. ;  they  are  absorbed  from  the 
tissues  by  the  blood  and  conveyed  to  the  excretory  organs,  lungs, 
kidneys,  etc. 

Gases. — Oxygen,  nitrogen,  and  carbonic  acid  exist  in  varying 
proportions. 

BLOOD-CORPUSCLES. 

The  corpuscular  elements  of  the  blood  occur  under  two  distinct 
forms,  which,  from  their  color,  are  known  as  the  red  and  white 
corpuscles. 

The  red  corpuscles  as  they  float  in  a  thin  layer  of  the  liquor 
sanguinis  are  of  a  pale  straw-color ;  it  is  only  when  aggregated  in 
masses  that  they  assume  the  bright  red  color.  In  form  they  are 
circular  and  biconcave ;  they  have  an  average  diameter  of  ^aVo" 
of  an  inch. 

In  mammals,  birds,  reptiles,  amphibia,  and  fish  the  corpuscles  vary 
in  size  and  number,  gradually  becoming  larger  and  less  numerous 
as  the  scale  of  animal  life  is  descended,  e.  g. : 

TABLE    SHOWING    COMPARATIVE    DIAMETER    OF    RED    CORPUSCLES. 

Mammals.  Birds.  Reptiles.  Amphibia.  Fish 

Man,                 g^  Eagle,  lg\5  Turtle,        T^T     Frog,  T1fo     Perch,     51feff 

Chimpanzee,  -^^  Owl,  y^g  Tortoise,     j^j    Toad,  JTfa^    Carp,       ^yW 

Orang,             ^y  Sparrow,  BI^ff  Lizard,        „'„     Proteus,  ^v     pike»        W<yo 

Dog",                 s*w  Swallow,  yfa  Viper,          ^y.5     Siren,  ^      Eel,          T71J5 

Cat,                  ¥?V?  Pigeon,  „>,,                                    Amphiuma,  g£3 

H°g,                JsVo  Turkey,  ^\-s 

Horse,             ZSQV  Goose,  j^gg 

Ox>                   iisW  Swan,  j^s 

In  man  and  the  mammals  the  red  corpuscles  present  neither  a 
nucleus  nor  a  cell  wall,  and  are  universally  of  a  small  size.  They  can 
be  .readily  distinguished  from  the  corpuscles  of  birds,  reptiles,  and 
fish,  in  which  animals  they  are  larger,  oval  in  shape,  and  possess  a 
well-defined  nucleus. 


126  HUMAN    PHYSIOLOGY. 

The  red  corpuscles  are  exceedingly  numerous,  amounting  to  about 
5,000,000  in  a  cubic  millimeter  of  blood.  In  structure  they  consist 
of  a  firm,  elastic,  colorless  framework, — the  stroma, — in  the  meshes 
of  which  is  entangled  the  coloring-matter —  the  hemoglobin. 

CHEMIC    COMPOSITION    OF    RED    CORPUSCLES. 

Water 688.00 

Globulin 282.22 

Hemoglobin >. 16.75 

Fatty  matter 2.31 

Extractives 2.60 

Mineral    salts 8.12 


1,000.00 

Hemoglobin,  the  coloring-matter  of  the  corpuscles,  is  an  albu- 
minous compound,  composed  of  C,  O,  H,  N,  S,  and  iron.  It  may 
exist  in  either  an  amorphous  or  a  crystalline  form.  When  deprived 
of  all  its  oxygen,  except  the  quantity  entering  into  its  intimate  com- 
position, the  hemoglobin  becomes  purplish  in  color,  and  is  known  as 
reduced  hemoglobin.  When  exposed  to  the  action  of  oxygen,  it 
again  absorbs  a  definite  amount  and  becomes  scarlet  in  color,  and 
is  known  as  oxy hemoglobin.  The  amount  of  oxygen  absorbed  is  1.76 
c.c.  (  T7^  of  a  cubic  inch)  for  i  mg.  ( -^  of  a  grain)  of  hemoglobin. 

It  is  this  substance  which  gives  the  color  to  the  venous  and  arterial 
blood.  As  the  venous  blood  passes  through  the  capillaries  of  the 
lungs  the  reduced  hemoglobin  absorbs  the  oxygen  from  the  pul- 
monary air  and  becomes  oxyhemoglobin,  scarlet  in  color ;  the  blood 
becomes  arterial.  When  the  arterial  blood  passes  into  the  systemic 
capillaries,  the  oxygen  is  absorbed  by  the  tissues ;  the  hemoglobin 
becomes  reduced,  purple  in  color,  and  the  blood  becomes  venous. 
A  dilute  solution  of  oxyhemoglobin  gives  two  absorption  bands 
between  the  lines  D  and  E  of  the  solar  spectrum.  Reduced  hemo- 
globin gives  but  one  absorption  band,  occupying  the  space  existing 
between  the  two  bands  of  the  oxyhemoglobin  spectrum. 

The  function  of  the  red  corpuscle  is,  therefore,  to  absorb  oxygen 
and  carry  it  to  the  tissues ;  the  smaller  the  corpuscles  and  the 
greater  the  number,  the  greater  is  the  quantity  of  oxygen  absorbed, 
and,  consequently,  all  the  vital  functions  of  the  body  become  more 
active. 


BLOOD.  127 

The  white  corpuscles  are  far  less  numerous  than  the  red,  the  pro- 
portion being,  on  an  average,  about  i  white  to  from  350  to  400  red; 
they  are  globular  in  shape,  and  are  ^'su'o  °^  an  inch  in  diameter,  and 
consist  of  a  soft,  granular,  colorless  substance,  containing  several 
nuclei. 

The  white  corpuscles  possess  the  pawer  of  spontaneous  movement, 
alternately  contracting  and  expanding,  throwing  out  processes  of  their 
substance  and  quickly  withdrawing  them,  thus  changing  their  shape 
from  moment  to  moment.  These  movements  resemble  those  of  the 
ameba,  and  for  this  reason  are  termed  ameboid.  The  white  cor- 
puscles also  possess  the  capability  of  moving  from  place  to  place. 
In  the  interior  of  the  vessels  they  adhere  to  the  inner  surface, 
while  the  red  corpuscles  move  through  the  center  of  the  stream. 

The  white  corpuscles  are  identical  with  the  leukocytes,  and  are 
found  in  milk,  lymph,  chyle,  and  other  fluids. 

Origin  of  Corpuscles. — The  red  corpuscles  take  their  origin  from 
the  mesoblastic  cells  in  the  vascular  area  of  the  developing  embryo. 

In  the  adult  they  are  produced  from  colorless,  nucleated  corpuscles 
known  as  erythroblasts.  The  spleen  is  the  organ  in  which  they  are 
finally  destroyed. 

The  white  corpuscles  originate  from  the  lymphocytes  of  the 
adenoid  tissue. 

COAGULATION    OF    THE   BLOOD. 

When  blood  is  withdrawn  from  the  body  and  allowed  to  remain 
at  rest,  it  becomes  somewhat  thick  and  viscid  in  from  three  to 
five  minutes ;  this  viscidity  gradually  increases  until  the  entire 
volume  of  blood  assumes  a  jelly-like  consistence,  which  process 
occupies  from  five  to  fifteen  minutes. 

As  soon  as  coagulation  is  completed,  a  second  process  begins, 
which  consists  in  the  contraction  of  the  coagulum  and  the  oozing 
of  a  clear,  straw-colored  liquid, — the  serum, — which  gradually  in- 
creases in  quantity  as  the  clot  diminishes  in  size,  by  contraction, 
until  the  separation  is  completed,  which  occupies  from  twelve  to 
twenty-four  hours. 

The  changes  in  the  blood  are  as  follows  : 

Before  coagulation. 


128  HUMAN    PHYSIOLOGY. 

(Liq.  Sanguinis,  ^  r  Water. 

Albumin, 
or  ^consisting  of  J 

1   Fibrinogen. 
Plasma,  J  [  Salts. 

Corpuscles,  red  and  white. 
After  coagulation. 

"  •assamentum.  |  c  Fibrin. 

Clot  or  coagulum,          /  1  Corpuscles. 

Dead    blood.     J  f  Water. 

Serum,  containing       -s   Albumin. 

[  Salts. 

The  serum,  therefore,  differs  from  the  liquor  sanguinis  in  not 
containing  fibrin. 

In  from  twelve  to  twenty-four  hours  the  upper  surface  of  the  clot 
presents  a  grayish  appearance, — the  buffy  coat, — owing  to  the  rapid 
sinking  of  the  red  corpuscles  beneath  the  surface,  permitting  the 
fibrin  to  coagulate  without  them ;  this  substance  then  assumes  a 
grayish-yellow  tint.  Inasmuch  as  the  white  corpuscles  possess  a 
lighter  specific  gravity  than  the  red,  they  do  not  sink  so  rapidly, 
and,  becoming  entangled  in  the  fibrin,  assist  in  forming  the  buffy 
coat.  Continued  contraction  gives  a  cupped  appearance  to  the 
surface  of  the  clot. 

Inflammatory  states  of  the  blood  produce  a  marked  increase  in  the 
buffed  and  cupped  condition,  on  account  of  the  aggregation  of  the 
corpuscles  and  their  tendency  to  rapid  sinking. 

Nature  of  Coagulation. — Coagulated  fibrin  does  not  preexist 
in  the  blood,  but  is  formed  at  the  moment  blood  is  withdrawn  from 
the  vessels.  According  to  Denis,  a  liquid  substance — plasmin — 
exists  in  the  blood,  which,  when  withdrawn  from  the  circulation, 
decomposes  into  fibrin  and  metalbumin. 

According  to  Schmidt,  fibrin  results  from  the  union  of  fibrino- 
plastin  (paraglobulin)  and  nbrinogen,  brought  about  by  the  presence 
of  a  third  substance,  the  fibrin-ferment. 

According  to  Hammersten  and  others,  the  fibrin  obtained  from 
the  blood  after  coagulation  comes  from  the  fibrinogen  alone,  the 
conversion  being  brought  about  by  the  presence  of  a  ferment  sub- 
stance, paraglobulin  in  this  case  having  nothing  to  do  with  the 
change.  This  view  is  supported  by  the  fact  that  the  quantity  of 
fibrin  obtained  from  the  blood  is  never  greater  than  the  quantity 


CIRCULATION    OF   THE   BLOOD.  129 

of  fibrinogen  previously  present.  The  origin  of  the  ferment  is 
obscure,  but  there  is  reason  to  believe  that  it  comes  from  the  injured 
vascular  coats  or  from  the  breaking  of  the  white  corpuscles. 

Conditions  Influencing  Coagulation. — The  process  is  retarded  by 
cold,  retention  within  living  vessels,  neutral  salts  in  excess,  inflam- 
matory conditions  of  the  system,  imperfect  aeration,  exclusion  from 
air,  etc. 

It  is  accelerated  by  a  temperature  of  100°  F.,  contact  with  air, 
rough  surfaces,  and  rest. 

Blood  coagulates  in  the  body  after  the  arrest  of  the  circulation 
in  the  course  of  twelve  to  twenty-four  hours  ;  local  arrest  of  the  cir- 
culation, from  compression  or  a  ligature,  will  cause  coagulation,  thus 
preventing  hemorrhages  from  wounded  vessels. 

The  composition  of  the  blood  varies  in  different  portions  of  the 
body.  The  arterial  differs  from  the  venous,  in  being  more  coagulable ; 
in  containing  more  oxygen  and  less  carbonic  acid ;  in  having  a 
bright  scarlet  color,  from  the  union  of  oxygen  with  hemoglobin, 
the  purple  hue  of  venous  blood  results  from  the  deoxidation  of  the 
coloring-matter. 

The  blood  of  the  portal  vein  differs  in  constitution,  according  to 
different  stages  of  the  digestive  process ;  during  digestion  it  is 
richer  in  water,  albuminous  matter,  and  sugar ;  occasionally  it  con- 
tains fat ;  corpuscles  are  diminished,  and  there  is  an  absence  of 
biliary  substances. 

The  blood  of  the  hepatic  vein  contains  a  larger  proportion  of  red 
and  white  corpuscles ;  the  sugar  is  augmented,  while  albumin,  fat 
and  fibrin  are  diminished. 


CIRCULATION    OF    THE    BLOOD. 

The  circulatory  apparatus  by  which  the  blood  is  distributed  to 
all  portions  of  the  body  consists  of  a  central  organ, — the  heart, — 
with  which  is  connected  a  system  of  closed  vessels  known  as 
arteries,  capillaries,  and  veins.  Within  this  system  the  blood  is 
kept,  by  the  action  of  the  heart,  in  continual  movement,  distributing 
nutritive  matter  to  all  portions  of  the  body  and  carrying  waste 
matters  from  the  tissues  to  the  various  eliminating  organs. 

The  heart  is  a  hollow,  muscular  organ,  pyramidal  in  shape,  meas- 
10 


130  HUMAN   PHYSIOLOGY. 

tiring  about  5J4  inches  in  length  and  about  3^  in  breadth,  weighing 
from  10  to  12  ounces  in  the  male  and  from  8  to  10  in  the  female. 
Situated  in  the  thoracic  cavity,  between  the  lungs,  its  base  is  directed 
upward,  backward,  and  to  the  right ;  its  apex  is  directed  downward 
and  to  the  left. 

Pericardium. — The  heart  is  surrounded  by  a  closed  fibrous  mem- 
brane called  the  pericardium.  The  inner  surface  of  this  membrane 
is  lined  by  a  serous  membrane,  which  is  also  reflected  over  the 
surface  of  the  heart ;  between  the  two  surfaces  of  the  serous  mem- 
brane is  found  a  small  quantity  of  fluid  (the  pericardial  fluid),  which 
lubricates  the  surfaces  and  prevents  friction  during  the  movements 
of  the  heart.  The  interior  of  the  heart  is  also  lined  by  a  serous 
membrane,  called  the  endocardium. 

Cavities  of  the  Heart. — The  general  cavity  of  the  heart  is  sub- 
divided by  a  longitudinal  septum  into  a  right  and  left  half;  each  of 
these  cavities  is  in  turn  subdivided  by  a  transverse  constriction  into 
two  smaller  cavities,  which  communicate  with  each  other  and  are 
known  as  the  auricles  and  ventricles,  the  orifice  between  the  auricle 
and  ventricle  being  known  as  the  auriculoventricular  orifice.  The 
heart,  therefore,  consists  of  four  cavities — a  right  auricle  and  ven- 
tricle and  a  left  auricle  and  ventricle. 

Into  the  right  auricle  the  two  terminal  trunks  of  the  venous  sys- 
tem— the  superior  and  inferior  vena  caves — empty  the  venous  blood 
which  has  been  collected  from  all  parts  of  the  system ;  from  the  right 
ventricle  arises  the  pulmonary  artery,  which,  passing  into  the  lungs, 
distributes  the  blood  to  the  walls  of  the  air-cells  of  the  lungs ;  into 
the  left  auricle  empty  four  pulmonary  veins,  which  have  collected 
the  blood  from  the  lung  capillaries  ;  from  the  left  ventricle  springs 
the  aorta,  the  general  trunk  of  the  arterial  system,  the  branches  of 
which  distribute  the  blood  to  the  entire  system. 

The  Valves  of  the  Heart. — The  valves  of  the  heart  are  formed 
by  a  reduplication  of  the  endocardium  strengthened  by  connective 
tissue.  At  the  auriculoventricular  openings  on  the  right  and  left 
sides  of  the  heart,  respectively,  are  found  the '  tricuspid  and  mitral 
valves.  The  tricuspid  valve  consists  of  three,  the  mitral  of  two, 
cusps  or  segments,  which  project  into  the  interior  of  the  ventricle 
when  it  does  not  contain  blood.  At  their  bases  the  segments  are 
united  so  as  to  form  an  annular  membrane  attached  to  the  margin 
of  the  orifice.  To  the  free  edges  of  the  valves  are  attached  numer- 


CIRCULATION    OF  THE  BLOOD. 


131 


I 


ous  fine  threads, — the  chorda  tendinece, — which  are  the  tendons 
of  the  small  papillary  muscles  springing  from  the  walls  of  the 
ventricles. 

The  Semilunar  Valves. — At  the  openings  of  the  pulmonary  artery 
and  the  aorta  are   found  three   cup-shaped  or  semilunar  valves,  the 
free    edges     of    which     are     directed 
away      from     the      interior      of     the 
heart.     The  anatomic  arrangement  of 
the    valves    is    such    that    upon    their 
closure   regurgitation   of  the   blood   is 
prevented. 

The  Course  of  the  Blood  through 
the  Heart. — Reference  to  figure  17 
will  make  it  clear  that  there  is  a 
pathway  for  the  blood  between  the 
venae  cavse  on  the  right  side  and 
the  aorta  on  the  left  side  by  way 
of  the  right  side  of  the  heart,  the 
cardio-pulmonary  vessels  and  the  left 
side  of  the  heart. 

The  venous  blood  flowing  towards 
the  heart  is  emptied  by  the  su- 
perior and  inferior  venae  cavae  into 
the  right  auricle  from  which  it 
passes  through  the  auriculoventric- 
ular  opening  into  the  right  ven- 
tricle; thence  into  and  through  the 
pulmonary  artery  and  its  branches 
to  the  pulmonary  capillaries  where 
it  is  arterialized,  *.  e.}  yields  up  its 
carbon  dioxid  and  takes  on  a  fresh 
supply  of  oxygen — and  is  changed  in 
color  from  dark  blue  to  scarlet  red. 
The  arterialized  blood  flowing  to- 
wards the  heart  is  emptied  by  the 
pulmonary  veins  into  the  left  au- 
ricle from  which  it  passes  through 
the  auriculoventricular  opening  into 
the  left  ventricle;  thence  into  the 


FIG.  17. — SCHEME  OF  THE  CIR- 
CULATION.—  (Landois.) 

a.  Right,  b.  left,  auricle.  A. 
Right,  B.  left  ventricle,  i. 
Pulmonary  artery.  2.  Aorta. 
1.  Area  of  pulmonary,  K.  area 
of  systemic,  circulations,  o. 
The  superior  vena  cava.  G. 
Area  supplying  the  inferior 
vena  cava,  u.  d,  d.  Intes- 
tine, m.  Mesenteric  artery, 
q.  Portal  vein.  L.  Liver, 
h.  Hepatic  vein. 


132  HUMAN   PHYSIOLOGY. 

aorta  and  its  branches  to  the  systemic  capillaries  where  it  is  de- 
arterialized  by  an  opposite  exchange  of  gases,  *'.  e.,  yields  up  a 
portion  of  its  oxygen  to,  and  absorbs  carbon  dioxid  from  the  tissues, 
and  changes  in  color  from  scarlet  to  dark  blue.  The  venous  blood 
is  again  returned  to  the  systemic  veins  to  the  venae  cavae. 

While  there  is  but  one  circulation,  physiologists  frequently  divide 
the  circulatory  apparatus  into — 

1.  The    systemic    circulation,    which    includes    the    movement    of    the 
blood   from   the   left  side  of  the  heart  through   the   aorta   and   its 
branches,  through  the  capillaries  and  veins,  to  the  right  side. 

2.  The    pulmonary    circulation,    which    includes    the    course    of    the 
blood  from  the  right  side  through  the  pulmonary  artery,   through 
the  capillaries  of  the  lungs  and  pulmonary  veins,  to  the  left  side 
of  the  heart. 

3.  The  portal  circulation,  which  includes  the  portal  vein.     This  vein 
is  formed  by  the  union  of  the  radicles  of  the  gastric,  mesenteric, 
and   splenic   veins,    and   carries   the   blood   directly   into    the   liver, 
where   the   vein   divides   into   a   fine   capillary   plexus,   from   which 
the    hepatic    veins    arise ;    these    empty    into    the    ascending    vena 
cava. 

The  Mechanism  of  the  Heart. — The  immediate  cause  of  the 
movement  of  the  blood  through  the  blood-vessels  is  the  alternate 
contraction  and  relaxation  of  the  muscular  walls  of  the  heart,  and 
more  especially  the  walls  of  the  ventricles,  each  of  which  plays 
alternately  the  part  of  a  force  pump  and  to  a  slight  extent  of  a 
suction  pump.  The  motive  power  is  furnished  by  the  heart  itself. 
The  contraction  of  any  part  of  the  heart  is  termed  the  systole,  the 
relaxation,  the  diastole;  as  each  side  of  the  heart  has  two  cavities, 
the  walls  of  which  contract  and  relax  in  succession,  it  is  customary 
to  speak  of  an  auricular  systole  and  diastole  and  a  ventricular  systole 
and  diastole ;  as  the  two  sides  are  in  the  same  physiologic  relation 
they  contract  and  relax  in  the  same  periods  of  time. 

Movements  of  the  Heart. — At  each  beat,  during  the  systole,  the 
heart  hardens  and  becomes  shortened  in  its  long  diameter  ;  its  apex 
is  raised  up,  rotated  on  its  axis  from  left  to  right,  and  thrown  for- 
ward against  the  walls  of  the  chest.  The  impulse  of  the  heart,  ob- 
served about  two  inches  below  the  nipple  and  one  inch  to  the  sternal 
side,  between  the  fifth  and  sixth  ribs,  is  caused  mainly  by  the  apex 
of  the  heart  striking  against  the  chest  walls,  assisted  by  the  dis- 
tention  of  the  great  vessels  about  the  base  of  the  heart. 


CIRCULATION    OF   THE  BLOOD.  133 

The  Cardiac  Cycle. — The  entire  period  of  the  heart's  pulsation 
may  be  divided  into  three  stages,  viz. : 

1.  The  auricular  contraction  and  relaxation. 

2.  The  ventricular  contraction  and  relaxation. 

3.  The  pause  or  period  of  repose  during  which  both  auricles  and  ven- 
tricles   are   at   rest.      These    three    stages    constitute    collectively    a 
cardiac  cycle  or  a  cardiac  revolution. 

The  duration  of  a  cycle,  as  well  as  the  duration  of  its  three  stages, 
varies  in  different  animals  in  accordance  with  the  number  of  cycles 
which  recur  in  a  minute.  In  human  beings  in  adult  life  there  are 
about  72  cycles  to  the  minute ;  the  average  duration  is,  therefore, 
0.80  sec.  From  this  it  follows  that  the  time  occupied  by  any  one  of 
the  three  stages  must  be  extremely  short  and  difficult  of  determina- 
tion. From  experiments  on  animals  and  from  observations  made  on 
human  beings,  the  following  estimates  have  been  made  and  accepted 
as  approximately  correct  for  human  beings  : 

1.  The  auricular  systole — 0.16  sec.;  the  auricular  diastole,  0.64  sec. 

2.  The  ventricular  systole — 0.32  sec. ;  the  ventricular  diastole,  0.48  sec. 

3.  The  period  of  rest  for  both  auricles  and  ventricles — 0.32  sec. 

The  Action  of  the  Valves. — The  forward  movement  of  the  blood 
is  permitted  and  regurgitation  prevented  by  the  alternate  action  of 
the  auriculoventricular  and  semilunar  valves.  As  a  point  of  departure 
for  a  consideration  of  the  action  of  these  valves  and  their  relation 
to  the  systole  and  diastole  of  the  heart,  the  close  of  the  ventricular 
systole  may  be  selected.  At  this  moment,  the  semilunar  valves,  which 
during  the  systole,  are  directed  outward  by  the  blood  current  are 
now  suddenly  and  completely  closed  by  the  pressure  of  the  blood 
in  the  aorta  and  pulmonary  artery.  Regurgitation  into  the  ventricles 
is  thus  prevented. 

During  the  ventricular  systole,  the  relaxed  auricles  are  filling 
with  blood.  With  the  ventricular  diastole  this  blood  or  its  equivalent 
flows  into  the  relaxed  and  easily  distensible  ventricles  until  both 
auricles  and  ventricles  are  nearly  filled.  The  tricuspid  and  mitral 
valves  which  are  hanging  down  into  the  ventricular  cavities,  are  now 
floated  up  by  currents  of  blood  welling  up  behind  them  until  they 
are  nearly  closed.  The  auricles  now  suddenly  contract,  forcing 
their  contained  volumes  into  the  ventricles  which  become  fully 
distended. 

With  the  cessation  of  the  auricular  systole,  the  ventricular  systole 


134 


HUMAN   PHYSIOLOGY. 


begins.  If  the  blood  is  not  to  be  returned  to  the  auricles  the  tri- 
cuspid  and  mitral  valves  must  be  instantly"  and  completely  closed. 
This  is  accomplished  by  the  upward  pressure  of  the  blood  which 
brings  their  free  edges  in  close  apposition.  Reversal  of  these  valves 
is  prevented  by  the  contraction  of  the  papillary  muscles  which 
exert  a  traction  on  their  under  surfaces  and  edges  and  hold  them 
steady. 

The  blood  now  confined  in  the  ventricles  between  the  closed 
auriculoventricular  and  semilunar  valves  is  subjected  to  pressure 
on  all  sides ;  as  the  pressure  rises  proportionately  to  the  vigor  of 
the  contraction  there  comes  a  moment  when  the  intra-ventricular 
pressure  exceeds  that  in  the  aorta  and  pulmonary  artery ;  at  once  the 
semilunar  valves  are  thrown  open  and  the  blood  discharged.  Both 
contraction  and  outflow  continue  until  the  ventricles  are  practically 
empty,  when  relaxation  sets  in  attended  by  a  rapid  fall  of  pressure, 
under  the  influence  of  the  positive  pressure  of  the  blood  in  the 
aorta  and  pulmonary  artery,  the  semilunar  valves  are  again  closed. 
The  accumulation  of  blood  in  the  auricles,  attended  by  a  rise  in 
pressure,  again  forces  the  tricuspid  and  mitral  valves  open.  With 
these  events  the  cardiac  cycle  is  again  completed. 

Sounds  of  the  Heart. — If  the  ear  be  placed  over  the  cardiac 
region,  two  distinct  sounds  are  heard  during  each  revolution  of  the 
heart,  closely  following  each  other,  and  which  differ  in  character. 

The  sound  coinciding  with  the  systole  in  point  of  time — the  first 
sound — is  prolonged  and  dull,  and  caused  by  the  closure  and  vibra- 
tion of  the  auriculoventricular  valves,  the  contraction  of  the  walls 
of  the  ventricles,  and  the  apex-beat ;  the  second  sound,  occurring 
during  the  diastole,  is  short  and  sharp,  and  caused  by  the  closure 
of  the  semilunar  valves. 

The  frequency  of  the  heart's  action  varies  at  different  periods 
of  life,  but  in  the  adult  male  it  beats  about  seventy-two  times  a 
minute.  It  is  influenced  by  age,  exercise,  posture,  digestion,  etc. 

Age. — Before  birth,  the  number  of  pulsations  a  minute  averages  .  140 

During  the  first  year  it  diminishes  to .  .     .     .128 

During  the  third  year  it  diminishes  to 95 

From  the  eighth  to  the  fourteenth  year  averages    ....     84 
In  adult  life  the  average  is 72 


CIRCULATION    OF   THE   BLOOD.  135 

Exercise  and  digestion  increase  the  frequency  of  the  heart's  action. 

Posture  influences  the  number  of  pulsations  a  minute  ;  in  the  male, 
standing,  the  average  is  81  ;  sitting,  71  ;  lying,  66 — independent,  for 
the  most  part,  of  muscular  effort. 

The  force  exerted  by  the  left  ventricle  at  each  contraction  has 
been  estimated  at  fifty-two  pounds.  If  a  tube  be  inserted  into  the 
aorta,  the  pressure  there  will  be  sufficient  to  support  a  column  of 
blood  nine  feet,  or  a  column  of  mercury  six  inches,  in  height,  the 
weight  in  either  case  being  about  four  pounds.  The  estimation  of 
the  force  which  the  heart  is  required  to  exert  to  support  this  column 
of  blood  is  arrived  at  by  multiplying  the  pressure  in  the  aorta  (four 
pounds)  by  the  area  of  the  internal  surface  of  the  left  ventricle 
(about  thirteen  inches),  each  inch  of  the  ventricle  being  capable  of 
supporting  a  downward  pressure  of  four  pounds. 

Work  Done  by  the  Heart. — The  work  done  by  the  heart  is  esti- 
mated by  multiplying  the  amount  of  blood  sent  out  from  the  right 
and  left  ventricles  at  each  contraction  by  the  pressure  of  the  pul- 
monary artery  and  aorta,  respectively — e.  g.,  when  the  right  ventricle 
contracts,  it  forces  out  l/4  of  a  pound  of  blood,  and  in  so  doing 
must  overcome  a  pressure  in  the  pulmonary  artery  sufficient  to 
support  a  column  of  blood  three  feet  in  height ;  that  is,  must  exert 
energy  sufficient  to  raise  *4  of  a  pound  3  feet,  or  ^  X  3,  or  ^ 
of  a  pound  i  foot.  When  the  left  ventricle  contracts,  it  sends  out  ^4 
of  a  pound  of  blood,  and  in  so  doing  the  left  ventricle  must  overcome 
a  pressure  in  the  aorta  sufficient  to  support  a  column  of  blood  9  feet 
in  height ;  that  is,  must  exert  energy  sufficient  to  raise  *4  of  a  pound 
9  feet,  or  *4  X  9,  or  2^4  pounds  i  foot.  Work  done  is  estimated 
by  the  amount  of  energy  required  to  raise  a  definite  weight  a  definite 
height ;  the  unit,  the  foot-pound,  being  that  required  to  raise  i 
pound  i  foot. 

The  heart,  therefore,  at  each  systole  exerts  energy  sufficient  to 
raise  3  foot-pounds,  and  as  it  contracts  72  times  a  minute,  it  would 
raise  in  that  time  3  X  72,  or  216  foot-pounds;  and  in  one  hour 
216X60,  or  i2;96o  foot-pounds;  and  in  twenty-four  hours  12,960X24, 
or  311,040  foot-pounds,  or  138.5  foot-tons. 

The  Causation  of  the  Heart  Beat. — From  the  fact  that  the  heart 
will  continue  to  beat  for  a  variable  length  of  time  after  removal 
from  the  body  (the  time  varying  with  the  species  of  animal  from 
which  it  has  been  obtained)  it  is  evident  that  the  beat  is  independent 
of  the  central  nerve  system. 


136  HUMAN   PHYSIOLOGY. 

The  fundamental  condition  for  the  continuance  of  the  beat  is 
the  maintenance  of  the  irritability.  So  long  as  this  persists  the 
heart  will  respond  to  its  appropriate  stimulus.  The  irritability  of 
the  heart  within  the  body  is  dependent  on  the  supply  of  blood  coming 
through  its  nutrient  vessels  or  flowing  through  its  cavities.  Outside 
the  body,  the  irritability  can  be  maintained  for  some  hours  by  similar 
methods. 

The  Nature  of  the  Stimulus. — The  presence  of  nerve  cells  in  the 
walls  of  the  heart,  their  relation  to  the  muscle  cells,  the  pronounced 
activity  of  the  sinus  of  the  frog  heart  where  they  are  very  abundant ; 
the  feeble  activity  of  the  apex  where  they  are  absent  gave  rise  to  the 
idea  that  the  stimulus  is  a  nerve  impulse  rhythmically  and  auto- 
matically discharged  by  these  nerve  cells.  This  view  is  no  longer 
entertained.  It  has  been  demonstrated  that  portions  of  the  heart 
muscle,  that  do  not  contain  nerve  cells,  will  continue  to  exhibit 
rhythmic  contraction  for  some  hours  if  supplied  with  oxygenated  and 
defibrinated  blood ;  that  the  embryonic  heart  contracts  rhythmically 
before  nerve  cells  have  migrated  to  its  walls. 

The  stimulus  therefore  evidently  arises  within  the  heart  muscle. 
In  other  words,  it  is  my o genie  and  not  neurogenic  in  origin.  The 
stimulus  is  now  believed  to  be  chemic  in  character  and  due  to  a 
reaction  between  the  inorganic  salts  in  the  muscle  cells  and  those  in 
lymph  by  which  they  are  surrounded. 

The  Influence  of  the  Central  Nerve  System  on  the  Action  of 
the  Heart. — Though  the  heart  beat  is  independent  of  the  central 
nerve  system,  it  is  to  a  considerable  extent  modified  by  it  either 
in  the  way  of  inhibition  or  augmentation.  In  all  classes  of  animals 
the  heart  not  only  contains  localized  collections  of  nerve  cells,  but 
is  also  connected  with  the  central  nerve  system  by  two  sets  of 
nerve  fibers. 

In  the  frog  heart  a  group  of  nerve  cells  is  found  in  the  sinus  at 
its  junction  with  the  auricle,  and  known  as  the  crescent  or  ganglion 
of  Remak ;  a  second  group  is  found  at  the  base  of  the  ventricle  on 
its  anterior  aspect  and  known  as  the  ganglion  of  Bidder ;  a  third 
group  is  found  in  the  auricular  septum,  known  as  the  septal  ganglion, 
or  the  ganglion  of  Ludwig.  These  cells  were  formerly  regarded 
as  the  source  of  the  stimuli  for  the  heart's  contraction.  They  are 
regarded  now  as  trophic  in  function  and  influencing  in  some  way 
the  nutrition  of  the  heart  muscle. 


CIRCULATION   OF  THE  BLOOD.  137 

In  the  dog  and  the  mammalian  heart  generally,  the  nerve  cells 
though  present  are  not  arranged  in  such  definite  groups,  but  are  dis- 
tributed in  the  terminations  of  the  venae  cavae,  pulmonary  veins,  the 
walls  of  the  auricles  and  in  the  neighborhood  of  the  base  of  the 
ventricles. 

The  nerves  which  connect  the  heart  with  the  central  nerve 
system  are  the  pneumogastric,  or  vagus,  and  the  sympathetic. 

The  pneumogastric,  or  vagus  nerve,  close  to  its  connection  with  the 
medulla  oblongata,  receives  motor  nerves  from  the  spinal  accessory. 
It  also  contains  motor  fibers,  which  come  direct  from  the  medulla.  It 
then  passes  down  the  neck  and  enters  the  thorax.  Some  of  its 
fibers  join  the  cardiac  plexus  and  by  this  route  reach  the  heart. 
Experimental  evidence  indicates  that  the  terminal  fibers  of  the 
vagus  arborize  around  the  nerve  cells  in  the  heart  wall.  Feeble  stimu- 
lation of  the  trunk  of  the  vagus  is  followed  by  a  diminution  in  the 
rate  of  the  beat ;  strong  stimulation  is  followed  by  complete  cessa- 
tion or  inhibition  of  the  heart  beat,  the  organ  coming  to  rest  in  the 
condition  of  diastole.  Division  of  both  vagi,  in  the  dog,  at  a  time 
when  the  heart  is  beating  normally,  is  followed  by  a  considerable 
increase  in  the  frequency  of  the  beat.  For  these  reasons  the  vagus 
nerve  is  said  to  have  an  inhibitor  or  restraining  influence  on  the 
rate  of  the  heart  beat. 

The  sympathetic  nerves  are  derived  mainly  from  the  ganglion  stel- 
latum.  The  cells  of  this  ganglion,  however,  are  in  relation  with 
nerve  fibers  which  emerge  from  the  spinal  cord  in  the  second  and 
third  dorsal  nerves. 

Stimulation  of  the  sympathetic  fibers  beyond  the  ganglion  stellatum, 
is  followed  by  an  increase  in  the  rate  and  sometimes  by  an  increase 
in  the  force  of  the  heart  beat.  For  this  reason  the  sympathetic  is 
said  to  exert  an  accelerator  and  an  augmentor  influence  on  the  heart 
beat. 

ARTERIES. 

The  arteries  are  a  series  of  branching  tubes  conveying  blood  to 
all  portions  of  the  body.  They  are  composed  of  three  coats : 

1.  External,  formed  of  areolar  and  elastic  tissue. 

2.  Middle,   contains   both   elastic   and   muscle   fibers,    arranged   trans- 
versely to  the  long  axis  of  the  artery.     The  elastic  tissue  is  more 
abundant   in   the  larger   vessels,   the   muscular   in   the   smaller. 

3.  Internal,    composed    of    a    thin,    homogeneous    membrane,    covered 
with  a  layer  of  elongated  endothelial  cells. 


138  HUMAN   PHYSIOLOGY. 

The  arteries  possess  both  elasticity  and  contractility. 

The  property  of  elasticity  allows  the  arteries  already  full  to 
accommodate  themselves  to  the  incoming  amount  of  blood,  and  to 
convert  the  intermittent  acceleration  of  blood  in  the  large  vessels 
into  a  steady  and  continuous  stream  in  the  capillaries. 

The  contractility  of  the  smaller  vessels  equalizes  the  current  of 
blood,  regulates  the  amount  going  to  each  part,  and  promotes  the 
onward  flow  of  blood. 

Blood  Pressure. — The  immediate  cause  of  the  movement  of  the 
blood  from  the  beginning  of  the  aorta,  through  the  arteries,  capil- 
laries and  veins,  to  the  right  side  of  the  heart,  is  a  difference  of 
pressure  between  these  two  points.  A  corresponding  difference  of 
pressure  exists  between  the  beginning  of  the  pulmonary  artery  and 
the  left  side  of  the  heart.  To  this  pressure  the  term  blood  pressure 
is  given  and  may  be  denned  as  the  pressure  exerted  laterally  by 
the  moving  blood  stream  against  the  walls  of  the  arteries,  capillaries 
and  veins.  That  there  is  such  a  pressure  different  in  amount  in 
each  of  these  three  divisions  of  the  vascular  apparatus  is  evident  from 
the  results  which  follow  division  of  an  artery  or  a  vein  of  cor- 
responding size.  When  an  artery  is  divided  the  blood  spurts  from 
the  opening  for  a  considerable  distance  and  with  considerable 
velocity.  When  a  vein  is  divided  the  blood  as  a  rule  merely  wells 
out  of  the  opening  and-  with  but  slight  momentum.  These  results 
indicate  that  the  blood  exerts  a  greater  pressure  in  the  arteries 
than  in  the  veins.  Experimentally  it  has  been  shown  that  the 
pressure  is  greatest  in  the  aorta,  less  in  the  capillaries,  and  least  in 
the  veins.  The  pressure  in  the  aorta  expressed  in  millimeters  of 
mercury  is  about  160,  in  the  capillaries  35  to  20,  and  in  the  veins 
from  20  to  o  or  less  at  the  terminations  of  the  venae  cavse. 

The  causes  of  the  blood  pressure  are  first,  the  driving  power  of 
the  heart,  and  second,  the  resistance  offered  by  the  walls  of  the 
blood-vessels  to  the  flow  of  blood  through  them.  Owing  to  this 
resistance,  the  blood  has  accumulated  and  in  consequence  the  whole 
system  has  become  distended  by  the  lateral  pressure.  The  largest 
part  of  the  resistance,  however,  is  found  at  the  periphery  of  the 
arterial  system  and  is  partly  the  cause  of  the  high  pressure  in  the 
arteries. 

The  arterial  pressure  is  increased  or  decreased  by  influences  which 
act  upon  the  heart  or  upon  this  peripheral  resistance. 


CIRCULATION    OF  THE  BLOOD.  139 

If  while  the  force  of  the  heart  remains  the  same,  the  rate  in- 
creases, thus  increasing  the  volume  of  blood  in  the  arteries,  the 
pressure  rises.  If  the  rate  remains  the  same,  but  the  volume  of 
blood  discharged  increases,  the  pressure  will  also  rise.  If  the  pe- 
ripheral resistance  is  increased  by  contraction  of  the  arterioles  the 
pressure  rapidly  rises.  On  the  contrary,  a  diminution  in  the  rate 
and  force  of  the  heart  or  a  diminution  in  peripheral  resistance  by  a 
dilatation  of  the  arteries  cause  a  fall  in  pressure. 

The  Pulse. — The  pulse  may  be  denned  as  a  periodic  expansion 
and  recoil  of  the  arterial  system.  The  expansion  is  caused  by  the 
ejection  into  the  arteries  of  a  volume  of  blood  during  the  systole  ; 
the  recoil  is  due  to  the  reaction  of  the  arterial  walls  on  the  blood 
driving  it  forward  into  and  through  the  capillaries,  during  the 
diastole. 

At  the  close  of  the  cardiac  diastole  the  arteries  are  full  of  blood 
and  considerably  distended.  During  the  occurrence  of  the  succeed- 
ing systole,  the  incoming  volume  of  blood  is  accommodated  by  a 
movement  forward  of  a  portion  of  the  general  blood  volume  into 
the  capillaries  and  a  further  distention  of  the  arteries.  The  dis- 
tention  naturally  begins  at  the  beginning  of  the  aorta.  As  the  blood 
continues  to  be  discharged  from  the  heart,  adjoining  segments  of 
the  aorta  expand  in  quick  succession  and  by  the  end  of  the  systole 
the  expansion  has  travelled  over  the  arterial  system  as  far  as  the 
capillaries.  This  expansion  movement  which  passes  over  the  arterial 
system  in  the  form  of  a  wave  is  known  as  the  pulse  wave,  or  the 
pulse.  It  is  this  alternate  expansion  and  recoil  which  is  perceived 
by  the  finger  when  placed  over  the  course  of  an  artery.  The  artery 
best  adapted  for  this  purpose  is  the  radial  as  it  passes  across  the 
wrist  joint/ 

The  velocity  with  which  the  blood  flows  in  the  arteries  diminishes 
from  the  heart  to  the  capillaries,  owing  to  an  enlargement  in  the 
united  sectional  area  of  the  vessels  ;  the  velocity  increases  from  the 
capillaries  toward  the  heart  for  the  opposite  reason.  The  blood  moves 
most  rapidly  in  the  large  vessels,  and  especially  under  the  influence  of 
the  ventricular  systole.  From  experiments  on  animals,  it  has  been 
estimated  to  move  in  the  carotid  of  man  at  the  rate  of  sixteen  inches 
a  second,  and  in  the  large  veins  at  the  rate  of  four  inches  a  second. 

The  caliber  of  the  blood-vessels  is  regulated  by  the  vasomotor 
nerves,  which  have  their  origin  in  the  gray  matter  of  the  medulla 


140  HUMAN    PHYSIOLOGY. 

oblongata.  They  issue  from  the  spinal  cord  through  the  anterior 
roots  of  spinal  nerves,  pass  through  the  sympathetic  ganglia,  and 
ultimately  are  distributed  to  the  coats  of  the  blood-vessels.  They 
exert  at  different  times  a  constricting  and  a  dilating  action  upon 
the  vessels,  thus  keeping  up  the  arterial  tonus  and  the  average  blood 
pressure. 

Capillaries. — The  capillaries  constitute  a  network  of  vessels  of 
microscopic  size,  which  distribute  the  blood  to  the  inmost  recesses 
of  the  tissues,  inosculating  with  the  arteries  on  the  one  hand  and  the 
veins  on  the  other ;  they  branch  and  communicate  in  every  possible 
direction. 

The  diameter  of  a  capillary  vessel  varies  from  -Q-^Q-Q  to  joVo"  °^  an 
inch ;  the  walls  of  these  consist  of  a  delicate,  homogeneous  mem- 
brane, ^o'ff^o  °^  an  incn  in  thickness,  lined  by  flattened,  elongated, 
endothelial  cells,  between  which,  here  and  there,  are  observed 
stomata. 

The  rate  of  movement  in  the  capillary  vessels  is  estimated  at  one 
inch  in  thirty  seconds. 

In  the  capillary  current  the  red  corpuscles  may  be  seen  hurrying 
down  the  center  of  the  stream,  while  the  white  corpuscles  in  the 
still  layer  adhere  to  the  walls  of  the  vessel,  and  at  times  can  be 
seen  to  pass  through  the  walls  of  the  vessel  by  ameboid  movements. 

The  function  of  the  capillary  blood-vessel  is  to  permit  of  the 
passage  of  the  nutritive  materials  of  the  blood  out  into  the  tissue- 
spaces  and  the  passage  of  waste  products  from  the  tissue-spaces  into 
the  blood. 

The  passage  of  the  blood  through  the  capillaries  is  mainly  due 
to  the  force  of  the  ventricular  systole  and  the  elasticity  of  the 
arteries;  but  it  is  probably  also  aided  by  a  power  resident  in  the 
capillaries  themselves,  the  result  of  a  vital  relation  between  the  blood 
and  the  tissues. 

The  veins  are  the  vessels  which  return  the  blood  to  the  heart ; 
they  have  their  origin  in  the  venous  radicles,  and  as  they  approach 
the  heart  converge  to  form  larger  trunks,  and  terminate  finally  in 
the  venae  cavse. 

They  possess  three  coats — 

1.  External,  made  up  of  areolar  tissue. 

2.  Middle,    composed    of    non-striated    muscle-fibers ;    yellow,    elastic, 
and  fibrous  tissue. 


CIRCULATION    OF   THE   BLOOD.  141 

3.  Internal,  an  endothelial  membrane  similar  to  that  of  the  arteries. 
Veins  are  distinguished  by  the  possession  of  valves  throughout 
their  course,  which  are  arranged  in  pairs,  and  formed  by  a  reflec- 
tion of  the  internal  coat,  strengthened  by  fibrous  tissues ;  they 
always  look  toward  the  heart,  and  when  closed  prevent  a  reflux  of 
blood  in  the  veins.  Valves  are  most  numerous  in  the  veins  of  the 
extremities,  but  are  entirely  absent  in  many  others. 

The  onward  flow  of  blood  in  the  veins  is  mainly  due  to  the  action 
of  the  heart,  but  is  assisted  by  the  contraction  of  the  voluntary 
muscles  and  the  force  of  respiration. 

Muscular  contraction,  which  is  intermittent,  aids  the  flow  of  blood 
in  the  veins  by  compressing  them.  As  regurgitation  is  prevented  by 
the  closure  of  the  valves,  the  blood  is  forced  onward  toward  the 
heart. 

Rhythmic  movements  of  veins  have  been  observed  in  some  of  the 
lower  animals,  aiding  the  onward  current  of  blood. 

During  the  movement  of  inspiration  the  thorax  is  enlarged  in  all 
its  diameters,  and  the  pressure  on  its  contents  at  once  diminishes. 
Under  these  circumstances  a  suction  force  is  exerted  upon  the  great 
venous  trunks,  which  causes  the  blood  to  flow  with  increased  rapidity 
and  volume  toward  the  heart. 

Venous  Pressure. — As  the  force  of  the  heart-beat  is  nearly  ex- 
pended in  driving  the  blood  through  the  capillaries,  the  pressure  in 
the  venous  system  is  not  very  marked,  not  amounting  in  the  jugular 
vein  of  a  dog  to  more  than  y1^  that  of  the  carotid  artery. 

The  time  required  for  a  complete  circulation  of  the  blood  through- 
out the  vascular  system  has  been  estimated  to  be  from  twenty  to 
thirty  seconds,  while  for  the  entire  mass  of  blood  to  pass  through 
the  heart  fifty-eight  pulsations  would  be  required,  occupying  forty- 
eight  seconds. 

The  forces  keeping  the  blood  in  circulation  are : 

1.  Action  of  the  heart. 

2.  Elasticity  of  the  arteries. 

3.  Capillary  force. 

4.  Contraction  of  the  voluntary  muscles  upon  the  veins. 

5.  Respiratory  movements. 


142  HUMAN   PHYSIOLOGY. 

RESPIRATION. 

Respiration  is  the  function  by  which  oxygen  is  absorbed  into  the 
blood  and  carbonic  acid  exhaled.  The  assimilation  of  the  oxygen 
and  the  evolution  of  carbonic  acid  takes  places  in  the  tissues  as  a 
part  of  the  general  nutritive  process,  the  blood  and  respiratory  ap- 
paratus constituting  the  media  by  means  of  which  the  interchange 
of  gases  is  accomplished. 

The  respiratory  apparatus  consists  of  a  larynx,  trachea,  and  lungs. 

The  larynx  is  composed  of  firm  cartilages,  united  by  ligaments  and 
muscles.  Running  anteroposteriorly  across  the  upper  opening  are 
four  ligamentous  bands — the  two  superior  or  false  vocal  cords,  and 
the  two  inferior  or  true  vocal  cords, — formed  by  folds  of  the  mucous 
membrane.  They  are  attached  anteriorly  to  the  thyroid  cartilages 
and  posteriorly  to  the  arytenoid  cartilages,  and  are  capable  of  being 
separated  by  the  contraction  of  the  posterior  crico-arytenoid  muscles, 
so  as  to  admit  the  passage  of  air  into  and  from  the  lungs. 

The  trachea  is  a  tube  from  four  to  five  inches  in  length,  ^  of  an 
inch  in  diameter,  extending  from  the  cricoid  cartilage  of  the  larynx 
to  the  third  dorsal  vertebra,  where  it  divides  into  the  right  and  left 
bronchi.  It  is  composed  of  a  series  of  cartilaginous  rings,  which 
extend  about  two  thirds  around  its  circumference,  the  posterior  third 
being  occupied  by  fibrous  tissue  and  non-striated  muscle-fibers, 
which  are  capable  of  diminishing  its  caliber. 

The  trachea  is  covered  externally  by  a  tough,  fibro-elastic  mem- 
brane, and  internally  by  mucous  membrane,  lined  by  columnar, 
ciliated,  epithelial  cells.  The  cilia  are  always  waving  from  within 
outward.  When  the  two  bronchi  enter  the  lungs,  they  divide  and 
subdivide  into  numerous  smaller  branches,  which  penetrate  the 
lungs  in  every  direction  until  they  finally  terminate  in  the  pul- 
monary lobules. 

As  the  bronchial  tubes  become  smaller  their  walls  become  thinner ; 
the  cartilaginous  rings  disappear,  but  are  replaced  by  irregular  an- 
gular plates  of  cartilage ;  when  the  tube  becomes  less  than  J^  of  an 
inch  in  diameter,  they  wholly  disappear,  and  the  fibrous  and  mucous 
coats  blend,  forming  a  delicate  elastic  membrane,  with  circular 
muscle-fibers. 

The  lungs  occupy  the  cavity  of  the  thorax,  are  conic  in  shape,  of 
a  pink  color  and  a  spongy  texture.  They  are  composed  of  a  great 


RESPIRATION.  143 

number  of  distinct  lobules  (the  pulmonary  lobules'),  connected  by 
interlobular  connective  tissue.  These  lobules  vary  in  size,  are  of 
an  oblong  shape,  and  are  composed  of  the  ultimate  ramifications  of 
the  bronchial  tubes,  within  which  are  contained  the  air-vesicles  or. 
cells.  The  walls  of  the  air-vesicles,  exceedingly  thin  and  delicate, 
are  lined  internally  by  a  layer  of  tessellated  epithelium,  externally 
covered  by  elastic  fibers,  which  give  the  lungs  their  elasticity  and 
distensibility. 

The  venous  blood  is  distributed  to  the  lungs  for  aeration  by  the 
pulmonary  artery,  the  terminal  branches  of  which  form  a  rich  plexus 
of  capillary  vessels  surrounding  the  air-cells ;  the  air  and  blood  are 
thus  brought  into  intimate  relationship,  being  separated  only  by  the 
delicate  walls  of  the  air-cells  and  capillaries. 

The  thoracic  cavity,  in  which  the  respiratory  organs  are  lodged, 
is  of  a  conic  shape,  having  its  apex  directed  upward,  its  base  down- 
ward. Its  framework  is  formed  posteriorly  by  the  spinal  column, 
anteriorly  by  the  sternum,  and  laterally  by  the  ribs  and  costal  car- 
tilages. Between  and  over  the  ribs  lie  muscles,  fascia,  and  skin, 
above,  the  thorax  is  completely  closed  by  the  structures  passing 
into  it  and  by  the  cervical  fascia  and  skin  ;  below,  it  is  closed  by 
the  diaphragm.  It  is,  therefore,  an  air-tight  cavity. 

The  Pleura. — Each  lung  is  surrounded  by  a  closed  serous  mem- 
brane (the  pleura),  one  layer  of  which  (the  visceral}  is  reflected 
over  the  lung;  the  other  (the  parietal},  reflected  over  the  wall  of 
the  thorax ;  between  the  two  layers  is  a  small  amount  of  fluid,  which 
prevents  friction  during  the  play  of  the  lungs  in  respiration. 

Owing  to  the  elastic  tissue  which  is  present  in  the  lungs,  they  are 
very  readily  distensible ;  so  much  so,  indeed,  that  the  pressure  of 
the  air  inside  the  trachea  and  lungs  is  sufficient  to  distend  them 
until  they  completely  fill  all  parts  of  the  thoracic  cavity  not  occupied 
by  the  heart  and  great  vessels.  The  elastic  tissue  endows  them  not 
only  with  distensibility,  but  also  with  the  power  of  elastic  recoil, 
by  which  they  are  enabled  to  accommodate  themselves  to  all  varia- 
tions in  the  size  of  the  thoracic  cavity. 

When  the  chest-walls  recede,  the  air  within  the  lungs  expands  and 
presses  them  against  the  ribs ;  when  the  chest-walls  contract,  the  air 
being  driven  out,  the  elastic  tissue  recoils  and  the  lungs  return  to 
their  original  condition.  The  movements  of  the  lungs  are,  there- 
fore, entirely  passive. 


144  HUMAN   PHYSIOLOGY. 

As  the  capacity  of  the  chest  in  a  state  of  rest  is  greater  than 
the  volume  of  the  lungs  after  they  are  collapsed,  it  is  quite  evi- 
dent that  in  the  living  condition  the  lungs  are  distended  and  in  a 
state  of  elastic  tension,  which  is  greater  or  less  in  proportion  as 
the  thoracic  cavity  is  increased  or  diminished  in  size.  The  elastic 
tissue,  always  on  the  stretch,  is  endeavoring  to  pull  the  visceral 
layer  of  the  pleura  away  from  the  parietal  layer,  but  is  antagonized 
by  the  pressure  of  the  air  within  the  air-passages.  This  condition 
of  things  persists  as  long  as  the  thoracic  cavity  remains  air-tight ; 
but  if  an  opening  be  made  in  the  thoracic  wall,  the  pressure  of  the 
external  air,  which  was  previously  supported  by  the  practically  rigid 
walls  of  the  thorax,  now  presses  upon  the  lung  with  as  much  force 
as  the  air  within  the  lung.  The  two  pressures  being  neutralized, 
there  is  nothing  to  prevent  the  elastic  tissue  from  recoiling,  driving 
the  air  out,  and  collapsing.-  The  elastic  tension  of  the  lungs  can  be 
readily  measured  in  man  after  death  by  inserting  a  manometer  into 
the  trachea.  Upon  opening  the  thorax  and  allowing  the  tissue  to 
recoil,  the  air  passes  upon  the  mercury  and  elevates  it,  the  extent 
to  which  it  is  raised  being  the  index  of  the  pressure.  Hutchinson 
calculated  the  pressure  to  be  one  half  pound  to  the  square  inch  of 
lung  surface. 

Respiratory  Movements. — The  movements  of  respiration  are  two, 
and  consist  of  an  alternate  dilatation  and  contraction  of  the  chest, 
known  as  inspiration  and  expiration. 

1.  Inspiration   is   an   active  process,   the   result   of   the   expansion   of 
the   thorax,    whereby   the   atmospheric    air   is    introduced   into    the 
lungs. 

2.  Expiration  is  a  partially  passive  process,  the  result  of  the  recoil 
of  the   elastic   walls   of  the   thorax,   and   the   recoil   of  the   elastic 
tissue   of  the  lungs,   whereby   the   intrapulmonary   air   is   expelled. 

In  inspiration  the  chest  is  enlarged  by  an  increase  in  all  its 
diameters — viz. : 

1.  The  vertical  is   increased  by  the   contraction   and   descent   of   the 
diaphragm. 

2.  The    anteroposterior    and    transverse    diameters    are    increased    by 
the  elevation   and  rotation   of  the  ribs  upon   their   axes. 

In  ordinary  tranquil  inspiration  the  muscles  which  elevate  the  ribs 
and  thrust  the  sternum  forward,  and  so  increase  the  diameters  of 
the  chest,  are  the  external  intercostals,  running  from  above  downward 


RESPIRATION. 


145 


and  forward  ;  the  sternal  portion  of  the  internal  intercostals,  and  the 
levatores  costarum. 

In  the  extraordinary  efforts  of  inspiration  certain  auxiliary  muscles 
are   brought   into   play, — viz.,   the   sternomastoid,  pectorales,   serratus 
magnus, — which  increase  the  capac- 
ity of  the  thorax  to  its  utmost  limit. 

In  expiration  the  diameters  of  the 
chest  are  all  diminished — viz. : 

1.  The  vertical,  by  the  ascent  of  the 
diaphragm. 

2.  The  anteroposterior,  by  a  depres- 
sion of  the  ribs  and  sternum. 

In  ordinary  tranquil  expiration 
the  diameters  of  the  thorax  are 
diminished  by  the  recoil  of  the  elas- 
tic tissue  of  the  lungs  and  the 
ribs ;  but  in  forcible  expiration  the 
muscles  which  depress  the  ribs  and 
sternum,  and  thus  further  diminish 
the  diameter  of  the  chest,  are  the 
internal,  intercostals,  the  infracos- 
tals,  and  the  triangularis  sterni. 

In  the  extraordinary  efforts  of  ex- 
piration certain  auxiliary  muscles 
are  brought  into  play, — viz.,  the  ab- 
do.minal  and  sacrolumbalis  muscles, 
— which  diminish  the  capacity  of 
the  thorax  to  its  utmost  limit. 

Expiration  is  aided  by  the  recoil  of  the  elastic  tissue  of  the 
lungs  and  ribs  and  by  the  pressure  of  the  air. 

Movements  of  the  Glottis. — At  each  inspiration  the  rima  glottidis 
is  dilated  by  a  separation  of  the  vocal  cords,  produced  by  the  con- 
traction of  the  crico-arytenoid  muscles,  so  as  freely  to  admit  the 
passage  of  air  into  the  lungs ;  in  expiration  they  fall  passively 
together,  but  do  not  interfere  with  the  exit  of  air  from  the  chest. 

Nerve   Mechanism   of   Respiration. — The   movements   of   respira- 
tory muscles,   though  capable   of  being  modified  to   a  certain   extent 
by    efforts    of   the    will,    are    of   an    automatic    character,    and   called 
forth    by    nerve    impulses    emanating    from    the    medulla    oblongata. 
11 


FIG.  1 8. — DIAGRAM  OF  THE  RESPI- 
RATORY ORGANS. 

The  windpipe,  leading  down  from 
the  larynx,  is  seen  to  branch 
into  two  large  bronchi,  which 
subdivide  after  they  enter 
their  respective  lungs. 


146  HUMAN    PHYSIOLOGY. 

The  respiratory  center,  the  so-called  vital  point,  generates  the 
nerve  impulses,  which,  traveling  outward  through  the  phrenic  and 
intercostal  nerves,  excite  contractions  of  the  diaphragm  and  inter- 
costal muscles,  respectively.  This  center  is  for  the  most  part  auto- 
matic in  its  action,  though  it  is  capable  of  being  modified  by  im- 
pulses reflected  to  it  through  various  sensor  nerves. 
This  center  may  be  stimulated : 

1.  Directly,  by  the  condition  of  the  blood.     An  increase  of  carbonic 
acid  or  a  diminution  of  oxygen  in  the  blood  causes  an  acceleration 
of    the    respiratory    movements ;    the    reverse    of    these    conditions 
causes  a  diminution  of  the  respiratory  movements. 

2.  Indirectly,  by  reflex  action.     The  medulla  may  be  excited  to  action 
through  the  pneumogastric  nerve,  by  the  presence  of  carbonic  acid 
in   the   lungs    irritating   its    terminal    filaments ;    through   the   fifth 
nerve,   by   irritation   of   the   terminal   branches ;    and   through   the 
nerves  of  general  sensibility.      In   either  case  this   center  reflects 
motor   impulses   to    the    respiratory    muscles    through    the   phrenic, 
intercostal,  inferior  laryngeal,   and   other  nerves. 

Types  of  Respiration. — The  abdominal  type  is  most  marked  in 
young  children,  irrespective  of  sex,  the  respiratory  movements  being 
effected  by  the  diaphragm  and  abdominal  muscles, 

In  the  superior  costal  type,  exhibited  by  the  adult  female,  the 
respiratory  movements  are  more  marked  in  the  upper  part  of  the 
chest,  from  the  first  to  the  seventh  ribs,  permitting  the  uterus  to 
ascend  in  the  abdomen  during  pregnancy  without  interfering  with 
respiration. 

In  the  inferior  costal  type,  manifested  by  the  male,  the  move- 
ments are  largely  produced  by  the  muscles  of  the  lower  portions  of 
the  chest,  from  the  seventh  rib  downward,  assisted  by  the  diaphragm. 

The  respiratory  movements  vary  according  to  age,  sleep,  and  exer- 
cise, being  most  frequent  in  early  life,  but  averaging  twenty  a 
minute  in  adult  life.  They  are  diminished  by  sleep  and  increased 
by  exercise.  There  are  about  four  pulsations  of  the  heart  to  each 
respiratory  act. 

During  both  inspiration  and  expiration  two  sounds  are  produced : 
the  one,  heard  in  the  thorax,  in  the  trachea,  and  larger  bronchial 
tubes,  is  tubular  in  character ;  the  other,  heard  in  the  substance  of  the 
lungs,  is  vesicular  in  character, 


RESPIRATION.  147 

Amount  of  Air  Exchanged  in  Respiration,  and  Capacity  of  Lungs. 

The  tidal  or  breathing  volume  of  air,  that  which  passes  in  and 
out  of  the  lungs  at  each  inspiration  and  expiration,  is  estimated  at 
from  twenty  to  thirty  cubic  inches. 

The  complemental  air  is  that  amount  which  can  be  taken  into  the 
lungs  by  a  forced  inspiration,  in  addition  to  the  ordinary  tidal  vol- 
ume, and  amounts  to  about  no  cubic  inches. 

The  reserve  air  is  that  which  usually  remains  in  the  chest  after 
the  ordinary  efforts  of  expiration,  but  which  can  be  expelled  by 
forcible  expiration.  The  volume  of  reserve  air  is  about  100  cubic 
inches. 

The  residual  air  is  that  portion  which  remains  in  the  chest  and 
cannot  be  expelled  after  the  most  forcible  expiratory  efforts,  and 
which  amounts,  according  to  Dr.  Hutchinson,  to  about  100  cubic 
inches. 

The  vital  capacity  of  the  chest  indicates  the  amount  of  air  that 
can  be  forcibly  expelled  from  the  lungs  after  the  deepest  possible 
inspiration,  and  is  an  index  of  an  individual's  power  of  breathing 
in  disease  and  during  prolonged  severe  exercise.  The  combined 
amount  of  the  tidal,  the  complemental,  and  the  reserve  air,  230 
cubic  inches,  represents  the  vital  capacity  of  an  individual  five  feet 
seven  inches  in  height.  The  vital  capacity  varies  chiefly  with 
stature.  It  is  increased  eight  cubic  inches  for  every  inch  in  height 
above  this  standard,  and  diminishes  eight  cubic  inches  for  each  inch 
below  it. 

The  tidal  volume  of  air  is  carried  only  into  the  trachea  and  large 
bronchial  tubes  by  the  inspiratory  movements.  It  reaches  the 
deeper  portions  of  the  lungs  in  obedience  to  the  law  of  diffusion  of 
gases,  which  is  inversely  proportionate  to  the  square  root  of  their 
densities. 

The  ciliary  action  of  the  columnar  cells  lining  the  broncial  tubes 
also  assists  in  the  interchange  of  air  and  carbonic  acid. 

The  entire  volume  of  air  passing  in  and  out  of  the  thorax  in 
twenty- four  hours  is  subject  to  great  variation,  but  can  be  readily 
estimated  from  the  tidal  volume  and  the  number  of  respirations  a 
minute.  Assuming  that  an  individual  takes  into  the  chest  twenty 
cubic  inches  at  each  inspiration,  and  breathes  eighteen  times  a 
minute,  in  twenty-four  hours  there  would  pass  in  and  out  of  the 
lungs  518,400  cubic  inches,  or  300  cubic  feet. 


148  HUMAN   PHYSIOLOGY. 

Chemistry  of  Respiration. — As  the  inspired  air  undergoes  a 
change  in  composition  during  its  stay  in  the  lungs  which  renders  it 
unfit  for  further  respiration,  it  becomes  requisite,  for  the  correct 
understanding  of  respiration,  to  ascertain  the  composition  of  both 
inspired  and  expired  air. 

Composition  of  Air. — Chemic  analysis  has  shown  that  every 
100  volumes  of  air  contain  20.81  volumes  of  oxygen,  70^19  volumes 
of  nitrogen,  and  0.03  volume  of  carbonic  acid.  Aqueous  vapor  is 
also  present,  though  the  quantity  is  variable.  The  higher  the  tem- 
perature, the  greater  the  amount. 

The  changes  in  the  air  effected  by  respiration  are: 
Loss  of  oxygen,  to  the  extent  of  five  cubic  inches  per  100  of  air,  or 

one  in  twenty. 
Gain  of  carbonic  acid,  to  the  extent  of  4.66  cubic  inches  per  100  of 

air,  or  0.93  inch  in  twenty. 
Increase  of  water-vapor  and  organic  matter. 
Elevation  of  temperature. 

Increase,  and  at  times  decrease,  of  nitrogen. 
Gain  of  ammonia. 

The  total  quantity  of  oxygen  withdrawn  from  the  air  and  con- 
sumed by  the  body  in  twenty-four  hours  amounts  to  fifteen  cubic 
feet,  and  can  be  readily  estimated  from  the  amount  consumed  at 
each  respiration.  Assuming  that  one  cubic  inch  of  oxygen  remains 
in  the  lungs  at  each  respiration,  in  one  hour  there  are  consumed 
1080  cubic  inches,  and  in  twenty- four  hours  25,920  cubic  inches, 
or  fifteen  cubic  feet,  weighing  eighteen  ounces.  To  obtain  this 
quantity,  300  cubic  feet  of  air  are  necessary. 

The  quantity  of  oxygen  consumed  daily  is  subject  to  considerable 
variations.  It  is  increased  by  exercise,  digestion,  and  lowered  tem- 
perature, and  decreased  by  the  opposite  conditions. 

The  quantity  of  carbonic  acid  exhaled  in  twenty-four  hours  varies 
greatly.  It  can  be  estimated  in  the  same  way.  Assuming  that  an 
individual  exhales  0.93  +  cubic  inch  at  each  respiration,  in  one 
hour  there  are  eliminated  1008  cubic  inches,  and  in  twenty- four 
24,192  cubic  inches,  or  fourteen  .cubic  feet,  containing  seven  ounces 
of  pure  carbon. 

The  exhalation  of  carbonic  acid  is  increased  by  muscular  exer- 
cise, nitrogenous  food,  tea,  coffee,  and  rice,  age,  and  by  muscular 
development;  decreased  by  a  lowering  of  temperature,  repose,  gin 
and  brandy,  and  a  dry  condition  of  the  air. 


RESPIRATION.  149 

As  there  is  always  more  oxygen  consumed  than  carbonic  acid 
exhaled,  and  as  oxygen  unites  with  carbon  to  form  an  equal  volume 
of  carbonic  acid,  it  is  evident  that  a  certain  quantity  of  oxygen 
disappears  within  the  body.  In  all  probability  it  unites  with  the 
sulphur  hydrogen  of  the  food  to  form  water. 

The  amount  of  water  vapor  which  passes  out  of  the  body  with  the 
expired  air  is  estimated  at  from  one  to  two  pounds. 

The  organic  matter,  though  slight  in  amount,  gives  the  odor  to 
the  breath.  In  a  room  with  defective  ventilation  the  organic  matter 
accumulates  and  gives  rise  to  headache,  nausea,  drowsiness,  etc. 
Long-continued  breathing  of  such  air  produces  general  ill  health. 
It  is  not  so  much  the  presence  of  CO2  in  increased  amount  as  the 
presence  of  organic  matter  which  necessitates  thorough  ventilation. 

Condition  of  the  Gases  in  the  Blood. 

Oxygen  is  absorbed  from  the  lungs  into  the  arterial  blood  by  the 
coloring-matter,  hemoglobin,  with  which  it  exists  in  a  state  of  loose 
combination,  and  is  disengaged  during  the  process  of  nutrition. 

Carbonic  acid,  arising  in  the  tissues,  is.  absorbed  into  the  blood  in 
consequence  of  its  alkalinity,  where  it  exists  in  a  state  of  simple  solu- 
tion and  also  in  a  state  of  feeble  combination  with  the  carbonates, 
soda  and  potassa,  forming  the  bicarbonates. 

Nitrogen   is   simply   held   in   solution   in   the  plasma. 

Exchange  of  Gases  in  the  Air-cells. — From  the  difference  in  ten- 
sion of  the  oxygen  in  the  air-cells  (27.44  mm-  of  Hg)  and  of  the 
oxygen  in  the  venous  blood  (22  mm.  Hg),  and  from  the  difference 
of  the  carbonic  acid  tension  in  the  venous  blood  (41  mm.  Hg)  and 
in  the  air-cells  (27  mm.  Hg),  it  might  be  concluded  that  the  passage 
of  the  gases  is  due  solely  to  pressure.  The  absorption  of  oxygen, 
however,  does  not  follow  absolutely  the  law  of  pressure ;  that  chemic 
processes  are  involved  is  shown  by  the  union  of  oxygen  with  the 
hemoglobin  of  the  blood  corpuscles.  The  exhalation  of  CO2  is  also 
partly  a  chemic  process,  as  it  has  been  shown  that  the  quantity 
excreted  is  greatly  increased  when  oxygen  is  simultaneously  absorbed. 
Oxygen  not  only  favors  the  exhalation  of  loosely  combined  CO2,  but 
favors  the  expulsion  of  that  which  can  be  excreted  only  by  the  addi- 
tion of  acids  to  the  blood. 

Changes  in  the  Blood  during  Respiration. 

As  the  blood  passes  through  the  lungs  it  is  changed  in  color,  from 
the  dark  purple  of  venous  blood  to  the  bright  red  of  arterial  blood. 


150  HUMAN   PHYSIOLOGY. 

The  heterogeneous  composition  of  venous  blood  is  exchanged  for 
the  uniform  composition  of  the  arterial. 
It  gains  oxygen  and  loses  carbonic  acid. 
Its  coagulability  is  increased.     Temperature  is  diminished. 

Asphyxia. — If  the  supply  of  oxygen  to  the  lungs  be  diminished 
and  the  carbonic  acid  retained  in  the  blood,  the  normal  respiratory 
movements  cease  and  the  condition  of  asphyxia  ensues,  which  soon 
terminates  in  death. 

The  phenomena  of  asphyxia  are  violent  spasmodic  action  of  the 
respiratory  muscles  attended  by  convulsions  of  the  muscles  of  the 
extremities,  engorgement  of  the  venous  system,  lividity  of  the 
skin,  abolition  of  sensibility  and  reflex  action,  and  death. 

The  cause  of  death  is  a  paralysis  of  the  heart  from  overdistention 
by  blood.  The  passage  of  the  blood  through  the  capillaries  is  pre- 
vented by  contraction  of  the  smaller  arteries,  from  irritation  of  the 
vasomotor  center.  The  heart  is  enfeebled  by  a  want  of  oxygen  and 
inhibited  in  its  action  by  the  inhibitory  centers. 


ANIMAL    HEAT. 

The  functional  activity  of  all  the  organs  and  tissues  of  the  body 
is  attended  by  the  evolution  of  heat,  which  is  independent,  for  the 
most  part,  of  external  conditions.  Heat  is  a  necessary  condition 
for  the  due  performance  of  all  vital  actions ;  although  the  body  con- 
stantly loses  heat  by  radiation  and  evaporation,  it  possesses  the 
capability  of  renewing  it  and  of  maintaining  it  at  a  fixed  standard. 
The  normal  temperature  of  the  body  in  the  adult,  as  shown  by  means 
of  a  delicate  thermometer  placed  in  the  axilla,  ranges  from  97.25° 
F.  to  99.5°  F.,  though  the  mean  normal  temperature  is  estimated  by 
Wunderlich  at  98.6°  F. 

The  temperature  varies  in  different  portions  of  the  body  accord- 
ing to  the  extent  to  which  oxidation  takes  place,  being  highest  in  the 
muscles,  in  the  brain,  blood,  liver,  etc. 

The  conditions  which  produce  variations  in  the  normal  tempera- 
ture of  the  body  are :  age,  period  of  the  day,  exercise,  food  and 
drink,  climate,  season,  and  disease. 

Age. — At  birth  the  temperature  of  the  infant  is  about  i°  F.  above 
that  of  the  adult,  but  in  a  few  hours  falls  to  95.5°  F.,  to  be  followed 


ANIMAL   HEAT.  151 

in  the  course  of  twenty-four  hours  by  a  rise  to  the  normal  or  a  degree 
beyond.  During  childhood  the  temperature  approaches  that  of  the 
adult ;  in  aged  persons  the  temperature  remains  about  the  same, 
though  they  are  not  so  capable  of  resisting  the  depressing  effects  of 
external  cold  as  adults.  A  diurnal  variation  of  the  temperature 
occurs  from  1.8°  F.  to  3.7°  F.  (Jiirgensen)  ;  the  maximum  occurring 
late  in  the  afternoon,  from  4  to  9  P.  M.  ;  the  minimum,  early  in  the 
morning,  from  i  to  7  A.  M. 

Exercise. — The  temperature  is  raised  from  i°  to  2°  F.  during 
active  contractions  of  the  muscular  masses,  and  is  probably  due  to 
the  increased  activity  of  chemic  changes ;  a  rise  beyond  this  point 
being  prevented  by  its  diffusion  to  the  surface,  consequent  on  a 
more  rapid  circulation,  radiation,  more  rapid  breathing,  etc. 

Food  and  Drink. — The  ingestion  of  a  hearty  meal  increases  the 
temperature  but  slightly ;  an  absence  of  food,  as  in  starvation,  pro- 
duces a  marked  decrease.  Alcoholic  drinks,  in  large  amounts,  in 
persons  unaccustomed  to  their  use,  cause  a  depression  of  the  tem- 
perature amounting  to  from  i°  to  2°  F.  Tea  causes  a  slight  elevation. 

External  Temperature. — Long-continued  exposure  to  cold,  espe- 
cially if  the  body  is  at  rest,  diminishes  the  temperature  from  i°  to  2° 
F.,  while  exposure  to  a  great  heat  slightly  increases  it. 

Disease  frequently  causes  a  marked  variation  in  the  normal  tem- 
perature of  the  body,  which  rises  as  high  as  107°  F.  in  typhoid  fever 
and  105°  F.  in  pneumonia;  in  cholera  it  falls  as  low  as  80°  F. 
Death  usually  occurs  when  the  heat  remains  high  and  persistent, 
from  106°  to  110°  F. ;  the  increase  of  heat  in  disease  is  due  to 
excessive  production  rather  than  to  diminished  elimination. 

The  source  of  heat  is  to  be  sought  for  in  the  chemic  decomposi- 
tions and  hydrations  taking  place  during  the  general  process  of 
nutrition,  and  in  the  combustion  of  the  carbonaceous  compounds  by 
the  oxygen  of  the  inspired  air ;  the  amount  of  its  production  is  in 
proportion  to  the  activity  of  the  internal  changes. 

Every  contraction  of  a  muscle,  every  act  of  secretion,  each  exhibi- 
tion of  nerve  force,  is  accompanied  by  a  change  in  the  chemic  com- 
position of  the  tissues  and  an  evolution  of  heat.  The  reduction  of 
the  disintegrated  tissues  to  their  simplest  form  by  oxidation,  and 
the  combination  of  the  oxygen  of  the  inspired  air  with  the  carbon 
and  hydrogen  of  the  blood  and  tissues,  results  in  the  formation  of 
carbonic  acid  and  water  and  the  generation  of  a  great  amount  of 
heat. 


152  HUMAN   PHYSIOLOGY. 

•  Certain  elements  of  the  food,  particularly  the  non-nitrogenized 
substances,  undergo  oxidation  without  taking  part  in  the  formation 
of  the  tissues,  being  transformed  into  carbonic  acid  and  water,  and 
thus  increase  the  sum  of  heat  in  the  body. 

Heat-producing  Tissues. — All  the  tissues  of  the  body  add  to  the 
general  amount  of  heat,  according  to  the  degree  of  their  activity. 
But  special  structures,  on  account  of  their  mass  and  the  large  amount 
of  blood  they  receive,  are  particularly  to  be  regarded  as  heat  pro- 
ducers, e.  g. : 

1.  During  mental  activity  the  brain  receives  nearly  one  fifth  of  the 
entire  volume  of  blood,  and  the  venous  blood  returning  from  it  is 
charged  with  waste  matters,  and  its  temperature  is  increased. 

2.  The  muscular  tissue,  on  account  of  the  many  chemic  changes  oc- 
curring during  active  contractions,  must  be  regarded  as  the  chief 
heat-producing  tissue. 

3.  The  secreting  glands,  during  their  functional  activity,  add  largely 
to  the  amount  of  heat. 

The  entire  quantity  of  heat  generated  within  the  body  has  been 
demonstrated  experimentally  to  be  about  2,300  calories,  a  calory, 
or  heat  unit,  being  that  amount  of  heat  required  to  raise  the  tempera- 
ture of  one  kilogram  of  water  2.2  pounds  i°  C.  This  quantity  of 
heat,  if  not  utilized  and  retained  within  the  body,  would  elevate  its. 
temperature  in  twenty-four  hours  about  60°  F.  That  this  volume 
of  heat  depends  very  largely  upon  the  oxidation  of  the  food-stuffs 
can  be  shown  experimentally. 

The  normal  temperature  of  the  body  is  maintained  by  a  constant 
expenditure  of  the  heat  in  several  directions : 

1.  In  warming  the  food,  drink,  and  air  that  are  consumed  in  twenty- 
four  hours.     For  this  purpose  about  157  heat  units  are  required. 

2.  In    evaporating   water    from   the    skin    and   lungs,    619    heat   units 
being  utilized  for  this  purpose. 

3.  In   radiation  and  conduction.     By  these  processes  the  body  loses 
at  least  fifty  per  cent,  of  its  heat,  or  1,156  heat  units. 

4.  In  the  production  of  work  ;  the  work  of  the  circulatory,  respiratory, 
muscular,    and   nervous    apparatus   being  performed   by   the   trans- 
formation of  369  heat  units  into  units  of  work. 

The  nervous  system  influences  the  production  of  heat  in  a  part  by 
increasing  the  amount  of  blood  passing  through  it  by  its  action  upon 
the  vasomotor  nerves.  Whether  there  exists  a  special  heat-center 
has  not  been  satisfactorily  determined,  though  this  is  probable. 


SECRETION.  153 

SECRETION. 

The  process  of  secretion  consists  in  the  separation  of  materials 
from  the  blood  which  are  either  to  be  again  utilized  to  fulfil  some 
special  purpose  in  the  economy,  or  are  to  be  removed  from  the 
body  as  excrementitious  matter ;  in  the  former  case  they  constitute 
the  secretions,  in  the  latter,  the  excretions. 

The  materials  which  enter  into  the  composition  of  the  secretions 
are  derived  from  the  nutritive  principles  of  the  blood,  and  require 
special  organs — e.  g.,  gastric  glands,  mammary  glands,  etc. — for 
their  proper  elaboration. 

The  'materials  which  compose  the  excretions  preexist  in  the  blood, 
and  are  the  results  of  the  activities  of  the  nutritive  process ;  if 
retained  within  the  body,  they  exert  a  deleterious  influence  upon  the 
composition  of  the  blood. 

Destruction  of  a  secreting  gland  abolishes  the  secretion  peculiar  to 
it,  and  it  can  not  be  formed  by  any  other  gland ;  but  among  the 
excreting  organs  there  exists  a  complementary  relation,  so  that  if 
the  function  of  one  organ  be  interfered  with,  another  performs  it  to  a 
certain  extent. 

Classification  of  the  Secretions. 

PERMANENT    FLUIDS. 

Serous  fluids.  Vitreous  humor  of  the  eye. 

Synovial  fluid.  Fluid  of  the  labyrinth  of  the  in- 

Aqueous   humor   of  the   eye.  ternal  ear. 

Cerebro-spinal   fluid. 

TRANSITORY    FLUIDS. 

Mucus.  Pancreatic    juice. 

Sebaceous  matter.  Secretion  from  Brunner's  glands. 

Cerumen  (external  meatus).  Secretion     from     Lieberkiihn's 

Meibomian  fluid.  glands. 

Milk    and    colostrum.  Secretions    from   follicles   of  the 

Tears.  large  intestine. 

Saliva.  Bile    (also   an   excretion). 

Gastric  juice. 

EXCRETIONS. 

Perspiration  and  the  secretion  of       Urine. 

the   axillary   glands.  Bile  (also  a  secretion). 


154  HUMAN   PHYSIOLOGY. 

FLUIDS    CONTAINING    FORMED    ANATOMIC    ELEMENTS. 

Seminal  fluid,  containing  sperma-       Fluid  of  the  Graafian  follicles, 
tozoids. 

The  essential  apparatus  for  secretion  is  a  delicate,  homogeneous, 
structureless  membrane,  on  one  side  of  which,  in  close  contact,  is 
a  capillary  plexus  of  blood-vessels,  and  on  the  other  side  a  layer 
of  cells  the  physiologic  function  of  which  varies  in  different  situ- 
ations. 

Secreting  organs  may  be  divided  into  membranes  and  glands. 

Serous  membranes  usually  exist  as  closed  sacs,  the  inner  surfaces 
of  which  are  covered  by  pale,  nucleated  epithelium,  containing  a 
small  amount  of  secretion. 

The  serous  membranes  are  the  pleura,  peritoneum,  pericardium, 
synovial  sacs,  etc. 

The  serous  fluids  are  of  a  pale  amber  color,  somewhat  viscid, 
alkaline,  coagulable  by  heat,  and  resemble  the  serum  of  the  blood ; 
their  amount  is  but  small.  The  pleural  fluid  varies  from  four  to 
seven  drams ;  the  peritoneal  from  one  to  four  ounces ;  the  pericardial 
from  one  to  three  drams. 

The  synovial  fluid  is  colorless,  alkaline,  and  extremely  viscid, 
from  the  presence  of  synovin. 

The  function  of  serous  fluids  is  to  moisten  the  opposing  surfaces, 
so  as  to  prevent  friction  during  the  play  of  the  viscera. 

The  mucous  membranes  are  soft  and  velvety  in  character,  and  line 
the  cavities  and  passages  leading  to  the  exterior  of  the  body — e.  g., 
the  gastro -intestinal,  pulmonary  and  genito -urinary.  They  consist 
of  a  primary  basement  membrane  covered  with  epithelial  cells,  which 
in  some  situations  are  tessellated,  in  others,  columnar. 

Mucus  is  a  pale,  semitransparent,  alkaline  fluid,  containing  epi- 
thelial cells  and  leukocytes.  It  is  composed,  chemically,  of  water, 
an  albuminous  principle  (mucin),  and  mineral  salts;  the  principal 
varieties  are  nasal,  bronchial,  vaginal,  and  urinary. 

Secreting  glands  are  formed  of  the  same  elements  as  the  secreting 
membranes,  but  instead  of  presenting  flat  surfaces,  are  involuted, 
forming  tubules,  which  may  be  simple  follicles — e.  g.,  mucous,  uterine, 
or  intestinal ;  or  compound  follicles — e.  g.,  gastric  glands,  mammary 
glands,  or  racemose  glands — e.  g.,  salivary  glands  and  pancreas. 
They  are  composed  of  a  basement  membrane,  enveloped  by  a  plexus 
of  blood-vessels,  and  are  lined  by  epithelial  and  true  secreting  cells, 


MAMMARY   GLANDS.  155 

which   in   different  glands  possess   the   capability   of   elaborating  ele- 
ments characteristic  of  their  secretions. 

In  the  production  of  the  secretion  two  essentially  different  proc- 
esses are  concerned: 

1.  Chemic. — The    formation    and    elaboration    of    the    characteristic 
organic  ingredients  of  the  secreted  fluids — e.  g.,  pepsin,  pancrea,tin 
— take  place  during  the  intervals   of  glandular  activity,   as  a  part 
of   the   general    function   of   nutrition.      They   are    formed   by   the 
cells   lining   the   glands,    and   can   often   be   seen    in   their   interior 
with  the  aid  of  the  microscope — e.  g.,  bile  in  the  liver  cells,   fat 
in  the  cells  of  the  mammary  gland. 

2.  Physical. — Consisting    of    a    transudation    of    water    and    mineral 
salts  from  the  blood  into  the  interior  of  the  gland. 

During  the  intervals  of  glandular  activity  only  that  amount  of 
blood  passes  through  the  gland  sufficient  for  proper  nutrition ;  when 
the  gland  begins  to  secrete,  under  the  influence  of  an  appropriate 
stimulus,  the  blood-vessels  dilate  and  the  quantity  of  blood  becomes 
greatly  increased  beyond  that  flowing  to  the  gland  during  its 
repose. 

Under  these  conditions  a  transudation  of  water  and  salt  takes 
place,  washing  out  the  characteristic  ingredients,  which  are  dis- 
charged by  the  gland  ducts.  The  discharge  of  the  secretions  is 
intermittent ;  they  are  retained  in  the  glands  until  they  receive  the 
appropriate  stimulus,  when  they  pass  into  the  larger  ducts  by  the 
vis  a  tergo,  and  are  then  discharged  by  the  contraction  of  the  muscu- 
lar walls  of  the  ducts. 

The  activity  of  glandular  secretion  is  hastened  by  an  increase  in 
the  blood-pressure  and  retarded  by  a  diminution. 

The  nerve   centers   in   the   medulla   oblongata   influence   secretion : 

1.  By    increasing    or    diminishing    the    amount    of    blood    entering    a 
gland. 

2.  By  exerting  a  direct  influence  upon  the  secreting  cells  themselves, 
the  centers  being  excited  by  reflex  irritation,  mental  emotion,  etc. 


MAMMARY    GLANDS. 

The  mammary  glands,  which  secrete  the  milk,  are  two  more  or 
less  hemispheric  organs,  situated  in  the  human  female  on  the 
anterior  surface  of  the  thorax.  Though  rudimentary  in  childhood, 


156  HUMAN    PHYSIOLOGY. 

they    gradually    increase    in    size    as    the    young    female    approaches 
puberty. 

The  gland  presents  at  its  convexity  a  small  prominence  of  skin 
(the  nipple),  which  is  surrounded  by  a  circular  area  of  pigmented 
skin  (the  areola).  The  gland  proper  is  covered  by  a  layer  of  adipose 
tissue  anteriorly  and  is  attached  posteriorly  to  the  pectoral  muscles 
by  a  meshwork  of  fibrous  tissue.  During  utero-gestation  the  mam- 
mary glands  become  larger,  firmer,  and  more  lobulated ;  the  areola 
darkens  and  the  veins  become  more  prominent.  At  the  period  of 
lactation  the  gland  is  the  seat  of  active  histologic  and  physiologic 
changes,  correlated  with  the  production  of  milk.  At  the  close  of 
lactation  the  glands  diminish  in  size,  undergo  involution,  and  grad- 
ually return  to  their  original  non-secreting  condition. 

Structure  of  the  Mammary  Gland. — The  mammary  gland  con- 
sists of  an  aggregation  of  some  fifteen  or  twenty  lobes,  each  of 
which  is  surrounded  by  a  framework  of  fibrous  tissue.  The  lobe  is 
provided  with  an  excretory  duct,  which,  as  it  approaches  the  base 
of  the  nipple,  expands  to  form  a  sinus  or  reservoir,  beyond  which  it 
opens  by  a  narrowed  orifice  on  the  surface  of  the  nipple.  On  trac- 
ing the  duct  into  a  lobe,  it  is  found  to  divide  and  subdivide,  and 
finally  to  terminate  in  lobules  or  acini.  Each  acinus  consists  of  a 
basement  membrane,  lined  by  low  polyhedral  cells.  Externally  it  is 
surrounded  by  connective  tissue  supporting  blood-vessels,  lymphatics 
and  nerves. 

MILK. 

Milk  is  an  opaque,  bluish-white  fluid,  almost  inodorous,  of  a  sweet 
taste,  an  alkaline  reaction,  and  a  specific  gravity  of  1025  to  1040. 
When  examined  microscopically  it  is  seen  to  consist  of  a  clear 
fluid  (the  milk-plasma),  holding  in  suspension  an  enormous  number 
of  small,  highly  refractive  oil-globules,  which  measure,  on  an 
average,  To1y^  °^  an  ^ncn  *n  diameter.  Each  globule  is  supposed 
by  some  observers  to  be  surrounded  by  a  thin,  albuminous  envelope, 
which  enables  it  to  maintain  the  discrete  form.  The  quantity  of  milk 
secreted  daily  by  the  human  female  averages  about  two  and  a  half 
pints.  The  milk  of  all  the  mammalia  consists  of  all  the  different 
classes  of  nutritive  principles,  though  in  varying  proportions.  The 
relative  proportions  in  which  these  constituents  exist  are  shown 
in  the  following  table  of  analyses  : 


MAMMARY   GLANDS. 
COMPOSITION    OF    MILK. 


157 


IN  xoo  PARTS. 

HUMAN. 

Cow. 

GOAT. 

Ass. 

SHEEP. 

MARE. 

Water. 

88.00 

86.87 

87.54 

91-57 

82.27 

88.80 

Caseinogen. 

2.40 

3.98 

3.00 

1.09 

6.10 

2.19 

Lactalbumin. 

0-57 

0.77 

0.62 

0.70 

1.  00 

0.42 

Fat. 

2.90 

3-50 

4.20 

1.02 

5-30 

2.50 

Lactose. 

5.87 

4.00 

4.00 

5-50 

4.20 

5.50 

Salts. 

o.  16 

0.17 

0.56 

0.42 

1.  00 

0.50 

Caseinogen  is  the  chief  proteid  constituent  of  milk,  and  is  held 
in  solution  by  the  presence  of  calcium  phosphate.  On  the  addition  of 
acetic  acid  or  of  sodium  chlorid  up  to  the  point  of  saturation,  the 
caseinogen  is  precipitated  as  such,  and  may  be  collected  by  appro- 
priate chemic  methods.  When  taken  into  the  stomach  caseinogen  is 
coagulated — that  is,  it  is  separated  into  casein  or  tyrein  and  a 
small  quantity  of  a  new  soluble  proteid.  The  ferment  which  induces 
this  change  is  known  as  rennin.  The  presence  of  calcium  phosphate 
is  necessary  for  this  coagulation. 

The  fat  of  milk  is  more  or  less  solid  at  ordinary  temperatures.  It 
is  a  composition  of  olein,  palmitin,  and  stearin,  with  a  small  quan- 
tity of  butyrin  and  caproin.  When  milk  is  allowed  to  stand  for 
some  time  the  fat-globules  rise  to  the  surface  and  form  a  thick  layer, 
known  as  cream.  When  subjected  to  the  churning  process,  the  fat 
globules  run  together  and  form  a  cohesive  mass — the  butter. 

Lactose  is  the  particular  form  of  sugar  characteristic  of  milk.  It 
belongs  to  the  saccharose  group  and  has  the  following  composition : 
CiaHasOn.  In  the  presence  of  the  bacillus  acidi  lactici  the  lactose 
is  decomposed  into  lactic  acid  and  carbon  dioxid,  the  former  of 
which  will  cause  a  coagulation  of  the  caseinogen. 


158  HUMAN   PHYSIOLOGY. 

Mechanism  of  Secretion. — During  the  time  of  lactation  the  mam- 
mary gland  exhibits  periods  of  secretory  activity  which  alternate 
with  periods  of  rest.  Coincidentally  with  these  periods,  certain  his- 
tologic  changes  take  place  in  the  secreting  structures  of  the  gland. 
At  the  close  of  a  period  of  active  secretion  each  acinus  presents  the 
following  features :  the  epithelial  cells  are  short,  cubic,  nucleated, 
and  border  a  relatively  wide  lumen  in  which  is  to  be  found  a 
variable  quantity  of  non-discharged  milk.  After  the  gland  has  rested 
for  some  time,  active  metabolism  again  begins.  The  epithelial 
cells  grow  and  elongate  ;  the  nucleus  divides  into  two  or  three  new 
nuclei,  and  at  the  same  time  the  cell  becomes  constricted;  the  inner- 
portion  is  detached  and  is  discharged  into  the  lumen.  Coincidentally 
with  these  changes  oil-globules  make  their  appearance  in  the  cell 
protoplasm,  some  of  which  are  discharged  separately  into  the  lumen, 
while  others  remain  for  a  time  associated  with  the  detached  cell. 
From  these  histologic  changes  it  would  appear  that  the  caseinogen 
and  the  fat-globules  are  metabolic  products  of  the  cell  protoplasm, 
and  not  derived  directly  from  the  blood.  That  lactose  has  a  similar 
origin  appears  certain  from  the  fact  that  it  is  formed  independently 
of  carbohydrate  food.  The  water  and  inorganic  salts  are  doubtless 
secreted  by  a  mechanism  similar  to  that  of  all  other  secreting 
glands. 


VASCULAR    OR    DUCTLESS    GLANDS. 
INTERNAL    SECRETIONS. 

The  metabolism  of  the  body  generally,  as  well  as  that  of  individual 
organs,  has  been  shown  to  be  related  not  only  to  the  physiologic 
activity  of  such  organs  as  the  liver  and  pancreas,  but  also  to  the 
activity  of  the  so-called  vascular  or  ductless  glands.  The  influence 
of  the  pancreas  in  regulating  the  production  of  glycogen  by  the  liver, 
and  the  influence  of  the  liver  in  the  maintenance  of  the  general 
metabolism  through  the  production  of  glycogen  and  the  formation 
or  urea,  are  now  established  facts.  That  the  vascular  or  ductless 
glands  to  an  equal  extent,  though  perhaps  in  a  different  way,  as- 
sist in  the  maintenance  of  physiologic  processes,  appears  certain 
from  the  results  of  animal  experimentation.  The  explanation  given, 
and  generally  accepted  at  the  present  time,  for  the  influence  of  these 
glands  is  that  they  produce  specific  substances,  which  are  poured  into 


VASCULAR  OR   DUCTLESS   GLANDS.  159 

the  blood  or  lymph  and  carried  direct  to  the  tissues,  to  the  activities 
of  which  they  appear  to  be  essential;  for  without  these  substances 
the  nutrition  of  the  tissues  declines  and  in  a  short  time  a  fatal 
termination  ensues. 

Inasmuch  as  these  partly  unknown  substances  are  formed  by  cell 
activity  and  are  poured  into  the  interstices  of  the  tissues,  they  have 
been  termed  "  internal  secretions."  Though  the  term  internal  se- 
cretions is  applicable  to  all  substances  which  arise  in  consequence 
of  tissue  metabolism,  and  which  after  being  poured  into  the  blood, 
influence  in  varying  degrees  and  ways  physiological  processes,  yet 
the  term  in  this  connection  will  be  applied  only  to  the  secretions 
of  the  thyroid  gland,  hypophysis  cerebri,  and  adrenal  bodies. 

Thyroid  Gland. — The  thyroid  gland  or  body  consists  of  two 
lobes  situated  on  the  lateral  aspect  of  the  upper  part  of  the  trachea. 
Each  lobe  is  pyriform  in  shape,  the  base  being  directed  down- 
ward and  on  a  level  with  the  fifth  or  sixth  tracheal  ring.  The 
lobe  is  about  50  mm.  in  length,  20  mm.  in  breadth,  and  25  mm. 
in  thickness.  As  a  rule,  the  lobes  are  united  by  a  narrow  band  or 
isthmus  of  the  same  tissue.  In  color  the  gland  is  reddish,  and  it 
is  abundantly  supplied  with  blood-vessels  and  lymphatics. 

Microscopic  examination  shows  that  the  thyroid  consists  of  an 
enormous  number  of  closed  sacs  or  vesicles,  variable  in  size,  the 
largest  not  measuring  more  than  o.i  mm.  in  diameter.  Each  sac  is 
composed  of-  a  thin  homogeneous  membrane  lined  by  cuboid  epi- 
thelium. The  interior  of  the  sac  in  adult  life  contains  a  trans- 
parent viscid  fluid,  containing  albumin  and  termed  "  colloid "  sub- 
stance. Externally,  the  sacs  are  surrounded  by  a  plexus  of  capillary 
blood-vessels  and  lymphatics.  The  individual  sacs  are  united  and 
supported  by  connective  tissue,  which  forms,  in  addition,  a  covering 
for  the  entire  gland. 

Function  of  the  Thyroid. — The  knowledge  at  present  possessed 
as  to  the  function  of  the  thyroid  gland,  especially  in  mammals,  is 
the  outcome  of  a  study  of  the  effects  which  follow  its  arrest  of  de- 
velopment in  the  child,  its  degeneration  in  the  adult,  its  extirpation 
in  the  human  being  as  well  as  in  animals.  The  results,  however, 
which  follow  its  extirpation  are  not  always  uniform  in  all  animals ; 
sufficient  reasons  for  which  lack  of  uniformity  can  not  always  be 
assigned. 

Cretinism,   a   condition   characterized   by   a   want   of  physical   and 


160  HUMAN    PHYSIOLOGY. 

mental  development,  is  associated  with,  if  not  directly  dependent  on, 
a  congenital  absence  or  an  arrested  development  of  the  thyroid, 
either  at  the  time  of  birth  or  during  the  early  years  of  childhood. 

Myxedema,  a  condition  of  the  skin  in  which  there  is  a  hyperplasia 
of  the  connective  tissue,  of  an  embryonic  type,  rich  in  mucin,  is 
generally  regarded  as  one  of  the  effects  of  degenerative  processes  in 
the  thyroid.  Partly  in  consequence  of  this  change  in  the  skin  the 
face  becomes  broader,  swollen,  and  flattened,  giving  rise  to  a  loss 
of  expression.  At  the  same  time  the  mind  becomes  dull,  clouded, 
even  approximating  the  idiotic  type.  This  supposed  infiltration  of 
the  skin  with  mucin  was  termed  myxedema  by  Ord,  who  at  the  same 
time  associated  it  with  a  change  in  the  structure  of  the  thyroid  as  a 
result  of  which  it  became  functionally  useless. 

Extirpation  of  the  thyroid,  for  relief  from  symptoms  due  to  grave 
pathologic  changes,  has  been  followed  in  human  beings  by  symptoms 
similar  to  those  of  myxedema.  To  this  condition  the  terms  operative 
myxedema  and  cachexia  strumipriva  have  been  applied. 

After  the  publication  of  the  history  of  the  myxedema  which  fol- 
lowed surgical  removal  of  the  thyroid,  Schiff,  in  1887,  repeated  his 
earlier  experiments  on  dogs,  and  found  again  that  removal  of  the 
thyroid  was  speedily  followed  by  tremors,  convulsions,  and  death. 
Similar  experiments  were  made  by  Horsley  on  monkeys,  with  results 
which  resembled  those  characteristic  of  myxedema.  Among  the 
symptoms  which  developed  within  a  few  days  after  the  removal  of 
the  gland  may  be  mentioned  loss  of  appetite ;  fibrillar  contractions 
of  muscles ;  tremors ;  spasms ;  mucinoid  degeneration  of  the  skin, 
giving  rise  to  puffiness  of  the  eyelids  and  face  and  to  a  swollen  con- 
dition of  the  abdomen ;  hebetude  of  mind,  frequently  terminating  in 
idiocy ;  fall  of  blood-pressure ;  dyspnea ;  albuminuria ;  atrophy  of  the 
tissues,  followed  by  death  of  the  animal  in  the  course  of  from  five  to 
eight  weeks.  The  complexus  of  symptoms  observed  in  monkeys  was 
divided  by  Horsley  into  three  stages  :  viz.,  the  neurotic,  the  mucinoid, 
and  the  atrophic.  It  is  evident  that  the  presence  of  the  thyroid  is 
essential  to  the  normal  activity  of  the  tissues  generally.  As  to  the 
manner  in  which  it  exerts  its  favorable  influence,  there  is  some  differ- 
ence of  opinion.  The  view  that  the  gland  removes  from  the  blood 
certain  toxic  bodies,  rendering  them  innocuous,  and  thus  preserving 
the  body  from  a  species  of  auto-intoxication,  is  gradually  yielding  to 
the  more  probable  view  that  the  epithelium  is  engaged  in  the 
secretion  of  a  specific  material,  which  finds  its  way  into  the  blood 


VASCULAR  OR   DUCTLESS    GLANDS.  161 

or  lymph  and  in  some  unknown  way  influences  favorably  tissue 
metabolism.  This  view  of  the  function  of  the  thyroid  is  supported 
by  the  fact  that  successful  grafting  of  a  portion  of  the  thyroid  be- 
neath the  skin  or  in  the  abdominal  cavity  will  prevent  the  usual 
symptoms  which  follow  thyroidectomy.  The  same  result  is  obtained 
by  the  intravenous  injection  of  thyroid  juice  or  by  the  administra- 
tion of  the  raw  gland.  It  was  shown  by  Murray  that  myxedematous 
patients  could  be  benefited,  and  even  cured,  by  feeding  them  with 
fresh  thyroids  or  even  with  the  dry  extract. 

The  chemic  features  of  the  material  secreted  and  obtained  from 
the  structures  of  the  thyroid  indicate  that  it  is  a  complex  proteid 
containing  iodin,  which,  under  the  influence  of  various  reagents, 
undergoes  cleavage,  giving  rise  to  a  non-proteid  residue,  which 
carries  with  it  the  iodin  and  phosphorus.  The  amount  of  iodin  in 
the  thyroid  varies  from  0.33  to  i  milligram  for  each  gram  of 
tissue.  To  this  compound  the  term  thyroiodin  has  been  given.  The 
administration  of  this  compound  produces  effects  similar  to  those 
which  follow  the  therapeutic  administration  of  the  fresh  thyroid 
itself :  viz.,  a  diminution  of  all  myxedematous  symptoms.  In  normal 
states  of  the  body,  thyroiodin  influences  very  actively  the  general 
metabolism.  It  gives  rise  to  a  decomposition  of  fats  and  proteids 
and  to  a  decline  in  body-weight.  In  large  doses  it  may  produce 
toxic  symptoms :  e.  g.}  increased  cardiac  action,  vertigo,  and  glyco- 
suria. 

The  Hypophysis  Cerebri.— This  is  a  small  body  lodged  in  the 
sella  turcica  of  the  sphenoid  bone.  In  consists  of  an  anterior  lobe, 
somewhat  red  in  color,  and  a  posterior  lobe,  yellowish-gray  in  color. 
The  former  is  much  the  larger  and  partly  embraces  the  latter.  The 
anterior  lobe  is  developed  from  an  invagination  of  the  epiblast  of 
the  mouth  cavity,  and  consists  of  distinct  gland  tissue.  The  posterior 
lobe  is  an  outgrowth  from  the  brain,  and  is  connected  with  the 
infundibulum  by  a  short  stalk.  It  has  been  suggested  that  the  term 
infundibular  body  be  reserved  for  the  posterior  lobe.  This  dis- 
tinction appears  to  be  desirable,  inasmuch  as  in  their  origin  and 
structure  they  are  separate  and  distinct  bodies. 

Removal  of  the  hypophysis  cerebri,  or  the  pituitary  body,  is  always 

followed  by  a   fatal  result,  preceded  by  symptoms   not  unlike   those 

which  follow  removal  of  the  thyroid  :  viz.,  anorexia,  tremors,  spasms, 

etc.     Degeneration  of  the  hypophysis  has  been  found  in  connection 

12 


162  HUMAN   PHYSIOLOGY. 

with  a  hypertrophic  condition  of  the  bones  of  the  face  and  extremi- 
ties, to  which  the  term  acromegalia  has  been  given. 

Intravenous  injection  of  an  extract  of  the  hypophysis  increases 
the  force  of  the  heart-beat  without  any  change  in  its  frequency,  and 
causes  a  rise  of  blood-pressure  from  a  stimulation  of  the  arterioles 
(Schafer  and  Oliver).  The  material  secreted  by  the  hypophysis  has 
not  been  isolated,  hence  its  chemic  features  are  unknown.  After  its 
formation  it  probably  passes  through  a  system  of  ducts  into  the 
cerebrospinal  fluid,  after  which  it  influences  the  metabolism  of  the 
nervous  and  osseous  tissues  as  well  as  the  force  of  the  heart  muscle. 

An  extract  of  the  hypophysis  itself  exerts  no  appreciable  effect 
on  the  blood-pressure  or  on  the  rate  of  the  heart-beat,  nor  does 
it  influence  the  circulatory  and  respiratory  organs  (Howell).  An 
extract  of  the  infundibular  body  intravenously  injected,  however, 
gives  rise  to  increased  blood-pressure  and  to  a  slowing  of  the 
heart-beat. 

Adrenal  Bodies,  or  Suprarenal  Capsules. — These  are  two  flat- 
tened bodies,  somewhat  crescentic  or  triangular  in  shape,  situated 
each  upon  the  upper  extremity  of  the  corresponding  kidney,  and  held 
in  place  by  connective  tissue.  They  measure  about  40  mm.  in 
height,  30  mm.  in  breadth,  and  from  6  to  8  mm.  in  thickness.  The 
weight  of  each  is  about  4  gm. 

Function  of  the  Adrenal  Bodies. — It  was  observed  by  Addison 
that  a  profound  disturbance  of  the  nutrition,  characterized  by  a 
bronze-like  discoloration  of  the  skin  and  mucous  membranes  of  the 
mouth,  extreme  muscular  weakness,  and  profound  anemia,  was  asso- 
ciated with,  if  not  dependent  on,  pathologic  conditions  of  the  supra- 
renal glands.  In  the  progress  of  the  disease  the  asthenia  gradually 
increases,  the  heart  becomes  weak,  the  pulse  small,  soft,  and  feeble, 
indicating  a  general  loss  of  tone  of  the  muscular  and  vascular  ap- 
paratus. Death  ensues  from  paralysis  of  the  respiratory  muscles. 
The  essential  nature  of  the  lesion  which  gives  rise  to  these  symp- 
toms has  not  been  determined. 

Removal  of  these  bodies  from  various  animals  is  invariably  and 
in  a  short  time  followed  by  death,  preceded  by  some  of  the  symp- 
toms characteristic  of  Addison's  disease.  Their  development,  how- 
ever, was  more  acute.  From  the  fact  that  animals  so  promptly 
die  from  extirpation  of  these  bodies,  and  the  further  fact  that  the 
blood  of  such  animals  is  toxic  to  those  the  subjects  of  recent  .extir- 


VASCULAR  OR   DUCTLESS   GLANDS.  Ib6 

pation,  but  not  to  normal  animals,  the  conclusion  was  drawn  that 
the  function  of  the  adrenal  bodies  was  to  remove  from  the  blood 
some  toxic  material  the  product  of  muscle  metabolism.  Its  accumu- 
lation after  extirpation  gives  rise  to  death  through  auto-intoxication. 

On  the  supposition  that  the  adrenals  might  secrete  and  pour  into 
the  blood  a  specific  material  which  favorably  influences  general 
metabolism,  Schafer  and  Oliver  injected  hypodermically  glycerin 
and  water  extracts,  and  observed  at  once  an  increased  activity  of 
the  heart-beats  and  of  the  respiratory  movements.  The  effects,  how- 
ever, were  only  transitory.  When  these  extracts  are  injected  into 
the  veins  directly,  there  follows  in  a  short  time  a  cessation  of  the 
auricular  contraction  of  the  heart,  though  the  ventricular  contrac- 
tion continues  with  an.  independent  rhythm.  If  the  vagi  are  cut 
previous  to  the  injection  or  if  the  inhibition  is  removed  by  atropin, 
the  rapidity  and  vigor  of  both  auricles  and  ventricles  are  increased. 
Whether  the  inhibitory  influence  is  removed  or  not,  there  is  a 
marked  increase  in  the  blood-pressure,  though  it  is  greater  in  the 
former  than  in  the  latter  instance.  This  is  attributed  to  a  direct 
stimulation  and  contraction  of  the  muscle-fibers  of  the  arterioles 
themselves,  and  not  to  vaso-motor  influences,  as  it  occurs  also  after 
division  of  the  cord  and  destruction  of  the  bulb.  The  contrac- 
tion of  the  arterioles  is  quite  general,  as  shown  by  plethysmographic 
studies  of  the  limbs,  spleen,  kidney,  etc.  Applied  locally  to  the 
mucous  membranes,  the  adrenal  extract  produces  contraction  of  the 
blood-vessels  and  pallor.  The  skeletal  muscles  are  affected  by  the 
extract  very  much  as  they  are  by  veratrin.  The  duration  of  a 
single  contraction  is  very  much  prolonged,  especially  in  the  phase  of 
relaxation  or  of  decreasing  energy. 

It  is  evident  from  these  experiments  that  the  adrenal  bodies  are 
engaged  in  elaborating  and  pouring  into  the  blood  a  specific  material 
which  stimulates  to  increased  activity  the  muscle-fibers  of  the  heart 
and  arteries,  and  thus  assists  in  maintaining  the  normal  blood- 
pressure  as  well  as  the  tonicity  of  the  skeletal  muscles.  The  active 
principle  of  this  gland  has  been  isolated  by  Abel  and  termed  epine- 
phrin. 


164  HUMAN    PHYSIOLOGY. 

EXCRETION. 

The  principal  excrementitious  fluids  discharged  from  the  body  are 
the  urine,  perspiration,  and  bile ;  they  hold  in  solution  principles 
of  waste  which  are  generated  during  the  activity  of  the  nutritive 
process  and  are  the  ultimate  forms  to  which  the  organic  constituents 
are  reduced  in  the  body.  They  also  contain  inorganic  salts. 

The  urinary  apparatus  consists  of  the  kidneys,  ureters,  and 
bladder. 

KIDNEYS. 

The  kidneys  are  the  organs  for  the  secretion  of  urine ;  they 
resemble  a  bean  in  shape,  are  from  four  to  five  inches  in  length, 
two  in.  breadth,  and  weigh  from  four  to  six  ounces. 

They  are  situated  in  the  lumbar  region,  one  on  each  side  of  the 
vertebral  column  behind  the  peritoneum,  and  extend  from  the 
eleventh  rib  to  the  crest  of  the  ilium ;  the  anterior  surface  is  convex, 
the  posterior  surface  concave,  the  latter  presenting  a  deep  notch, 
the  hilus. 

The  kidney  is  surrounded  by  thin,  smooth  membrane  composed  of 
white  fibrous  and  yellow  elastic  tissue  ;  though  it  is  attached  to  the 
surface  of  the  kidney  by  minute  processes  of  connective  tissue,  it 
can  be  readily  torn  away.  The  substance  of  the  kidney  is  dense, 
but  friable. 

Upon  making  a  longitudinal  section  of  the  kidney  it  will  be  ob- 
served that  the  hilus  extends  into  the  interior  of  the  organ  and  ex- 
pands to  form  a  cavity  known  as  the  sinus.  This  cavity  is  occupied 
by  the  upper,  dilated  portion  of  the  ureter,  the  interior  of  which 
forms  the  pelvis.  The  ureter  subdivides  into  several  portions,  which 
ultimately  give  origin  to  a  number  of  smaller  tubes,  termed  calyces, 
which  receive  the  apices  of  the  pyramids. 

The  parenchyma  of  the  kidney  consists  of  two  portions — viz. : 

1.  An    internal    or    medullary    portion,    consisting    of    a    series    of 
pyramids  or  cones,  some  twelve  or  fifteen  in  number.     They  pre- 
sent   a    distinctly    striated    appearance,    a    condition    due    to    the 
straight  direction  of  the  tubules  and  blood-vessels. 

2.  An   external   or   cortical  portion,   consisting   of   a    delicate   matrix 
containing  an  immense  number  of  tubules  having  a  markedly  con- 


KIDNEYS. 


165 


voluted  appearance.     Throughout  its  structure  are  found  numerous 
small  ovoid  bodies,  termed  Malpighian  corpuscles. 


FIG.  19. — LONGITUDINAL  SECTION  THROUGH  THE  KIDNEY,  THE  PELVIS  OF  THE 
KIDNEY,  AND  A  NUMBER  OF  RENAL  CALYCES. —  (Tyson,  after  Henle.) 

A.  Branch  of  the  renal  artery.  U.  Ureter.  C.  Renal  calyx,  i.  Cortex,  if. 
Medullary  rays.  i".  Labyrinth,  or  cortex  proper.  2.  Medulla.  2'.  Papil- 
lary portion  of  medulla,  or  medulla  proper.  2".  Border  layer  of  the 
medulla.  3,  3.  Transverse  section  through  the  axes  of  the  tubules  of 
the  border  layer.  4.  Fat  of  the  renal  sinus.  5,  5.  Arterial  branches. 
*  Transversely  coursing  medulla  rays. 

The  Uriniferous  Tubules. — The  kidney  is  a  compound,  tubular 
gland  composed  of  microscopic  tubules  whose  function  it  is  to 
secrete  from  the  blood  those  waste  products  which  collectively  con- 


166 


HUMAN   PHYSIOLOGY. 


stitute  the  urine.  If  the  apex  of  each  pyramid  be  examined  with 
a  lens,  it  will  present  a  number  of  small  orifices,  which  are  the 
beginnings  of  the  uriniferous  tubules.  From  this  point  the  tubules 
pass  outward  in  a  straight  but  somewhat  divergent  manner  toward 
the  cortex,  giving  off  at  acute  angles  a  number  of  branches  (Fig.  20). 
From  the  apex  to  the  base  of  the  pyra- 
mids they  are  known  as  the  tubules  of  Bel- 
^n*'  ^n  ^e  cortical  portion  of  the  kidney 
each  tubule  becomes  enlarged  and  twisted, 
and  after  pursuing  an  extremely  convo- 
luted course,  turns  backward  into  the  med- 
ullary portion  for  some  distance,  forming 
the  descending  limb  of  Henle's  loop  :  it  then 
turns  upon  itself,  forming  the  ascending 
limb  of  the  loop,  reenters  the  cortex, 
'  again  expands,  and  finally  terminates  in  a 
spheric  enlargement  known  as  Midler  s  or 
Bowman's  capsule.  Within  this  capsule 
is  contained  a  small  tuft  of  blood-vessels, 
constituting  the  glomerulus,  or  Malpighian 
corpuscle. 

Structure  of  the  Tubules.  —  Each  tubule 
consists  of  a  basement  membrane  lined  by 
epithelial  cells  throughout  its  entire  extent. 
The  tubule  and  its  contained  epithelium 

FIG.  20.  —  DIAGRAMMATIC      vary    in    shape    and    size    in    different    parts 
EXPOSITION    OF    THE  .  . 

METHOD    IN    WHICH      of     its     course.      Ihe     termination     of     the 

convoluted    tube     consists     of    a     little    sac 


THE      URINIFEROUS 

1  USES      UNITE      T  O 

FORM       PRIMITIVE      Or    capsule,    which    is    ovoid    in    shape    and 
CONES.  —  (Tyson, 
after  Ludwig.) 


. 

measures  about  ^  of  an  mch-  Thls  caP~ 
sule  is  lined  by  a  layer  of  flat- 
tened epithelial  cells,  which  is  also  reflected  over  the  surface  of  the 
glomerulus.  During  the  periods  of  secretory  activity  the  blood- 
vessels of  the  glomerulus  become  filled  with  blood,  so  that  the  cavity 
of  the  sac  is  almost  obliterated  ;  after  secretory  activity  the  blood- 
vessels contract  and  the  sac-cavity  becomes  enlarged.  In  that 
portion  of  the  tubule  lying  between  the  capsule  and  Henle's  loop  the 
epithelial  cells  are  cuboid  in  shape  ;  in  Henle's  loop  they  are  flat- 
tened, while  in  the  remainder  of  the  tubule  they  are  cuboid  and 
columnar. 


KIDNEYS.  167 

Blood-vessels  of  the  Kidney. — The  renal  artery  is  of  large  size 
and  enters  the  organ  at  the  hilum ;  it  divides  into  several  large 
branches,  which  penetrate  the  substance  of  the  kidney  between  the 
pyramids,  at  the  base  of  which  they  form  an  anastomosing  plexus, 
which  completely  surrounds  them.  From  this  plexus  vessels  follow 
the  straight  tubes  toward  the  apex,  while  others,  entering  the  corti- 
cal portion,  divide  into  small  twigs,  which  enter  the  Malpighian 
body  and  form  a  mass  of  convoluted  vessels,  the  glomerulus.  After 
circulating  through  the  Malpighian  tuft,  the  blood  is  gathered  to- 
gether by  two  or  three  small  veins,  which  again  subdivide  and  form 
a  fine  capillary  plexus,  which  envelops  the  convoluted  tubules ;  from 
this  plexus  the  veins  converge  to  form  the  emulgent  vein,  which 
empties  into  the  vena  cava. 

The  nerves  of  the  kidney  follow  the  course  of  the  blood-vessels 
and  are  derived  from  the  renal  plexus. 

The  ureter  is  a  membranous  tube,  situated  behind  the  peritoneum, 
about  the  diameter  of  a  goose-quill,  eighteen  inches  in  length,  and 
extends  from  the  pelvis  of  the  kidney  to  the  base  of  the  bladder, 
which  it  perforates  in  an  oblique  direction.  It  is  composed  of  three 
coats :  fibrous,  muscular,  and  mucous. 

The  bladder  is  a  reservoir  for  the  temporary  reception  of  the  urine 
prior  to  its  expulsion  from  the  body ;  when  fully  distended  it  is 
ovoid  in  shape,  and  holds  about  one  pint.  It  is  composed  of  four 
coats :  serous,  muscular  (the  fibers  of  which  are  arranged  longi- 
tudinally and  circularly),  areolar,  and  mucous.  The  orifice  of  the 
bladder  is  controlled  by  the  sphincter  vesicce,  a  muscular  band  about 
Y2  of  an  inch  in  width. 

As  soon  as  the  urine  is  formed  it  passes  through  the  tubuli 
uriniferi  into  the  pelvis  ,and  thence  through  the  ureters  into  the 
bladder,  which  it  enters  at  an  irregular  rate.  Shortly  after  a  meal, 
after  the  ingestion  of  large  quantities  of  fluid,  and  after  exercise,  the 
urine  flows  into  the  bladder  quite  rapidly,  while  it  is  reduced  to  a 
few  drops  during  the  intervals  of  digestion.  It  is  prevented  from 
regurgitating  into  the  ureters  by  the  oblique  direction  they  take 
between  the  mucous  and  muscular  coats. 

Nerve  Mechanism  of  Urination. — When  the  urine  has  passed 
into  the  bladder,  it  is  there  retained  by  the  sphincter  vesicae  muscle, 
kept  in  a  state  of  chronic  contraction  by  the  action  of  a  nerve  center 


168  HUMAN   PHYSIOLOGY. 

in  the  lumbar  region  of  the  spinal  cord.  This  center  can  be  in- 
hibited and  the  sphincte'r  relaxed,  either  reftexly,  by  impressions 
coming  through  sensory  nerves  from  the  mucous  membrane  of  the 
bladder,  or  directly,  by  a  voluntary  impulse  descending  the  spinal 
cord.  When  the  desire  to  urinate  is  experienced,  impressions  made 
upon  the  vesical  sensory  nerves  are  carried  to  the  centers  governing 
the  sphincter  and  detrusor  urince  muscles  and  to  the  brain.  If  now 
the  act  of  urination  is  to  take  place,  a  voluntary  impulse  originating 
in  the  brain  passes  down  the  spinal  cord  and  still  further  inhibits 
the  sphincter  vesicse  center,  with  the  effect  of  relaxing  the  muscle 
and  of  stimulating  the  center  governing  the  detrusor  muscle,  with 
the  effect  of  contracting  the  muscle  and  expelling  the  urine.  If 
the  act  is  to  be  suppressed,  voluntary  impulses  inhibit  the  detrusor 
center  and  possibly  stimulate  the  sphincter  center. 

The  genitospinal  center  controlling  these  movements  is  situated 
in  that  portion  of  the  spinal  cord  corresponding  to  the  origin  of  the 
third,  fourth,  and  fifth  sacral  nerves. 

URINE. 

Normal  urine  is  of  a  pale  yellow  or  amber  color,  perfectly  trans- 
parent, with  an  aromatic  odor,  an  acid  reaction,  a  specific  gravity  of 
1020,  and  a  temperature  when  first  discharged  of  100°  F. 

The  color  varies  considerably  in  health,  from  a  pale  yellow  to  a 
brown  hue,  owing  to  the  presence  of  the  coloring-matter,  urobilin  or 
urochrome. 

The  transparency  is  diminished  by  the  presence  of  mucus,  the 
calcium  and  magnesium  phosphates,  and  the  mixed  urates. 

The  reaction  of  the  urine  is  acid,  owing  to  the  presence  of  acid 
phosphate  of  sodium.  The  degree  of  acidity,  however,  varies  at 
different  periods  of  the  day.  Urine  passed  in  the  morning  is  strongly 
acid,  while  that  passed  during  and  after  digestion,  especially  if  the 
food  is  largely  vegetable  in  character,  is  either  neutral  or  alkaline. 

The  specific  gravity  varies  from   1015  to   1025. 

The  quantity  of  urine  excreted  in  twenty-four  hours  is  between 
forty  and  fifty  fluidounces,  but  ranges  above  and  below  this  standard. 

The  odor  is  characteristic,  and  caused  by  the  presence  of  taurylic 
and  phenylic  acids,  but  is  influenced  by  vegetable  foods  and  other 
substances  eliminated  by  the  kidneys. 


KIDNEYS. 


169 


967.0 
14.230 


10.635 


COMPOSITION    OF    URINE. 

Water 

Urea 

Other  nitrogenized  crystalline  bodies,  uric 
acid,  principally  in  the  form  of  alkaline 
urates, 

Creatin,  creatinin,  xanthin,  hypoxanthin, 

Hippuric  acid,  leucin,  tyrosin,  taurin,  cystin, 
all  in  small  amounts,  and  not  constant, 

Mucus  and  pigment, 


Salts 

Inorganic:  principally  sodium  and  potassium  ^ 
sulphates,  phosphates,   and   chlorids,   with 
magnesium  and  calcium  phosphates,  traces 
of  silicates  and  chlorids, 

Organic:  lactates,  hippurates,  acetates,   for- 
mates, which  appear  only  occasionally, 

Sugar a  trace 

Gases  (nitrogen  and  carbonic  acid  principally). 


8.135 


1,000.000 

The  average   quantity   of   the   principal   constituents    excreted   in 
twenty-four   hours   is    as    follows : 

Water 52.0  fluidounces. 

Urea        512.4  grains. 

Uric  acid 8.5          " 

Phosphoric  acid 45.0          " 

Sulphuric    acid 31.11        " 

Inorganic    salts 323.25        " 

Lime  and  magnesia 6.5          " 


To  determine  the  amount  of  solid  matters  in  any  given  amount 
of  urine,  multiply  the  last  two  figures  of  the  specific  gravity  by  the 
coefficient  of  Haeser,  2.33 — e.  g,,  in  1,000  grains  of  urine  having  a 
specific  gravity  1022,  there  are  contained  22X2.33  =  51.26  grains 
of  solid  matter. 


170  HUMAN   PHYSIOLOGY. 

Organic  Constituents  of  Urine.— Urea  is  one  of  the  most  im- 
portant of  the  organic  constituents  of  the  urine,  and  is  present  to 
the  extent  of  from  2.5  to  3.2  per  cent.  Urea  is  a  colorless,  neutral 
substance,  crystallizing  in  four-sided  prisms  terminated  by  oblique 
surfaces.  When  crystallization  is  caused  to  take  place  rapidly,  the 
crystals  take  the  form  of  long,  silky  needles.  Urea  is  soluble  in 
water  and  alcohol;  when  subjected  to  prolonged  boiling,  it  is  decom- 
posed, giving  rise  to  carbonate  of  ammonia.  •  In  the  alkaline  fer- 
mentation of  urine,  urea  takes  up  two  molecules  of  water  with  the 
production  of  carbonate  of  ammonia. 

The  average  amount  of  urea  excreted  daily  has  been  estimated 
at  about  500  grains.  As  urea  is  one  of  the  principal  products  of  the 
breaking  up  of  the  albuminous  •  compounds  within  the  body,  it  is 
quite  evident  that  the  quantity  produced  and  eliminated  in  twenty- 
four  hours  will  be  increased  by  any  increase  in  the  amount  of  al- 
buminous food  consumed,  or  by  a  rapid  destruction  of  albuminous 
tissues,  as  is  observed  in  various  pathologic  states,  inanition,  febrile 
conditions,  fevers,  etc.  A  farinaceous  or  vegetable  diet  will  diminish 
the  urea  production  nearly  one  half. 

Muscular  exercise  when  the  nutrition  of  the  body  is  in  a  state  of 
equilibrium  does  not  seem  to  increase  the  quantity  of  urea. 

Seat  of  Urea  Formation. — As  to  the  seat  of  urea  formation,  little 
is  positively  known.  It  is  quite  certain  that  it  preexists  in  the  blood 
and  is  merely  excreted  by  the  kidneys.  It  is  not  produced  in 
muscles,  as  even  after  prolonged  exercise  hardly  a  trace  of  urea  is 
to  be  found  in  them.  Experimental  and  pathologic  facts  point  to 
the  liver  as  the  probable  organ  engaged  in  urea  formation.  Acute 
yellow  atrophy  of  the  liver,  suppurative  diseases  of  the  liver,  diminish 
almost  entirely  the  production  01  urea. 

Uric  acid  is  also  a  constant  ingredient  of  the  urine  and  is  closely 
allied  to  urea.  It  is  a  nitrogen-holding  compound,  carrying  out  of  the 
body  a  portion  of  the  nitrogen.  The  amount  eliminated  daily  varies 
from  five  to  ten  grains.  Uric  acid  is  a  colorless  crystal  belonging 
to  the  rhombic  system.  It  is  insoluble  in  water,  and  if  eliminated 
in  excessive  amounts,  it  is  deposited  as  a  "  brick-red  "  sediment  in 
the  urine.  It  is  doubtful  if  uric  acid  exists  in  a  free  state,  being 
combined  for  the  most  part  with  sodium  and  potassium  bases  form- 
ing urates.  It  is  to  be  regarded  as  one  of  the  terminal  products 
of  the  decomposition  of  nucleic  acid  which  in  turn  is  derived  from 
nuclein,  a  constituent  of  cell  nuclei. 


KIDNEYS.  171 

Hippuric  acid  is  found  very  generally  in  urine,  though  it  is  present 
only  in  small  amounts.  It  is  increased  by  a  diet  of  asparagus,  cran- 
berries, plums,  and  by  the  administration  of  benzoic  and  cinnamic 
acids.  It  is  probably  formed  in  the  kidney. 

Kreatinin  resembles  the  kreatin  derived  from  muscles.  It  is  a 
colorless  crystal,  belonging  to  the  rhombic  system.  Its  origin  is 
unknown,  though  it  is  largely  increased  in  amount  by  albuminous 
food.  About  fifteen  grains  are  excreted  daily. 

Xanthin,  hypo-xanthin,  and  guanin  are  also  constituents  of 
urine.  They  are  nitrogenized  compounds  and  are  also  terminal 
products  of  nucleic  acid. 

Urobilin,  the  coloring-matter  of  the  urine,  is  a  derivative  of  the 
bile  pigments.  It  is  particularly  abundant  in  febrile  conditions,  giv- 
ing to  the  urine  its  reddish-yellow  color. 

Inorganic  Constituents  of  Urine. — Earthy  Phosphate.  Phos- 
phoric acid  in  combination  with  magnesium  and  calcium  is  excreted 
daily  to  the  extent  of  from  fifteen  to  thirty  grains.  The  phosphates 
are  insoluble  in  water,  but  are  held  in  solution  in  the  urine  by  its 
acid  ingredients,  alkalinity  of  the  urine  being  attended  with  a  copious 
precipitation  of  the  phosphates.  Mental  work  increases  the  amount 
of  phosphoric  acid  excreted,  a  condition  caused  by  increased  meta- 
bolism of  the  nervous  tissue. 

Sulphuric  acid  in  combination  with  sodium  and  potassium  con- 
stitutes the  sulphates,  of  which  about  thirty  grains  are  excreted 
daily.  Sulphuric  acid  results  largely  from  the  decomposition  of 
albuminous  food  and  from  increased  destruction  of  animal  tissues. 

The  gases  of  urine  are  carbonic  acid  and  nitrogen. 

Mechanism  of  Urinary  Secretion. — As  the  kidney  anatomically 
presents  an  apparatus  for  filtration  (the  Malpighian  bodies)  and  an 
apparatus  for  secretion  (the  epithelial  cells  of  the  urinary  tubules), 
it  might  be  inferred  that  the  elimination  of  the  constituents  of  the 
urine  is  accomplished  by  the  twofold  process  of  filtration  and 
secretion;  that  the  water  and  highly  diffusible  inorganic  salts 
simply  pass  by  diffusion  through  the  walls  of  the  blood-vessels  of 
the  glomerulus  into  the  capsule  of  Miiller,  while  the  urea  and  re- 
maining organic  constituents  are  removed  by  true  secretory  action 
of  the  renal  epithelium.  Modern  experimentation  supports  this  view 
of  renal  action. 


172  HUMAN   PHYSIOLOGY. 

The  secretion  of  urine,  is,  therefore,  partly  physical  and  partly 
vital. 

The  filtration  of  urinary  constituents  from  the  glomerulus  into 
Miiller's  capsule  depends  largely  upon  the  blood-pressure  and  the 
rapidity  of  blood  flow  in  the  renal  artery  and  glomerulus.  Among  the 
influences  which  increase  the  pressure  and  velocity  may  be  mentioned 
increased  frequency  and  force  of  the  heart's  action,  contraction  of 
the  capillary  vessels  of  the  body  generally,  dilatation  of  the  renal 
artery,  and  increase  in  the  volume  of  the  blood. 

The  reverse  conditions  lower  the  blood-pressure  and  diminish 
the  secretion  of  urine. 

The  fact  that  organic  matters  are  eliminated  by  the  secretory  ac- 
tivity of  the  renal  epithelium  seems  to  be  well  established  by 
modern  experiments.  These  substances,  removed  from  the  blood  in 
the  secondary  capillary  plexus  of  blood-vessels,  by  a  true  selective 
action  of  the  epithelium,  are  dissolved  and  washed  toward  the  pelves 
by  the  liquid  coming  from  the  capsules. 

The  blood-supply  to  the  kidney  is  regulated  by  the  nervous  system. 
If  the  renal  nerves  be  divided,  the  renal  artery  dilates  and  a  copious 
flow  of  urine  takes  place.  If  the  peripheral  ends  of  the  same 
nerves  be  stimulated,  the  artery  contracts  and  the  urinary  flow 
ceases.  The  same  is  true  of  the  splanchnic  nerves,  through  which  the 
vaso-motor  nerves  coming  from  the  medulla  oblongata  and  spinal 
cord  pass  to  the  renal  plexus. 


LIVER. 

The  liver  is  a  highly  vascular,  conglomerate  gland,  appended  to 
the  alimentary  canal.  It  is  the  largest  gland  in  the  body,  weighing 
about  four  and  one  half  pounds ;  it  is  situated  in  the  right  hypo- 
chondriac region,  and  is  retained  in  position  by  five  ligaments,  four 
of  which  are  formed  by  duplicatures  of  the  peritoneal  investment. 

The  proper  coat  of  the  liver  is  a  thin  but  firm  fibrous  membrane, 
closely  adherent  to  the  surface  of  the  organ,  which  it  penetrates  at 
the  transverse  fissure,  and  follows  the  vessels  in  their  ramifications 
through  its  substance,  constituting  Glissdn's  capsule. 

Structure  of  the  Liver. — The  liver  is  made  up  of  a  large  number 
of  small  bodies  (the  lobules},  rounded  or  ovoid  in  shape,  measuring 


LIVER.  173 

^5-  of  an  inch  in  diameter,  separated  by  a  space  in  which  are  situated 
blood-vessels,  nerves,  hepatic  ducts,  and  lymphatics. 

The  lobules  are  composed  of  cells,  which,  when  examined  micro- 
scopically, exhibit  a  rounded  or  polygonal  shape,  and  measure,  on 
the  average,  ToV^  of  an  inch  in  diameter ;  they  possess  one,  and 
sometimes  two,  nuclei ;  they  also  contain  globules  of  fat,  pigment 
matter,  and  animal  starch.  The  cells  constitute  the  secreting  struc- 
ture of  the  liver,  and  are  the  true  hepatic  cells. 

The  blood-vessels  which  enter  the  liver  are : 
i.  The  portal  vein,  made  up  of  the  gastric,  splenic,  and  superior  and 

inferior  me  sent  eric  veins. 
2    The  hepatic  artery,  a  branch  of  the  celiac  axis. 

Both  the  portal  vein  and  the  hepatic  artery  are  invested  by  a 
sheath  of  areolar  tissue. 

The  vessels  which  leave  the  liver  are  the  hepatic  veins,  originating 
in  its  interior,  collecting  the  blood  distributed  by  the  portal  vein 
and  hepatic  artery,  and  conducting  it  to  the  ascending  vena  cava. 

Distribution  of  Vessels. — The  portal  vein  and  the  hepatic  artery, 
upon  entering  the  liver,  penetrate  its  substance,  divide  into  smaller 
and  smaller  branches,  occupy  the  spaces  between  the  lobules,  com- 
pletely surrounding  and  limiting  them,  and  constitute  the  interlobular 
vessels.  The  hepatic  artery,  in  its  course,  gives  off  branches  to  the 
walls  of  the  portal  vein  and  Glisson's  capsule,  and  finally  empties 
into  the  small  branches  of  the  portal  vein  in  the  interlobular  spaces. 

The  interlobular  vessels  form  a  rich  plexus  around  the  lobules, 
from  which  branches  pass  to  neighboring  lobules  and  enter  their 
substance,  where  they  form  a  very  fine  network  of  capillary  vessels, 
ramifying  over  the  hepatic  cells,  in  which  the  various  functions  of  the 
liver  are  performed.  The  blood  is  then  collected  by  small  veins, 
converging  toward  the  center  of  the  lobule,  to  form  the  intralobular 
vein,  which  runs  through  its  long  axis  and  empties  into  the  sublobular 
vein.  The  hepatic  veins  are  formed  by  the  union  of  the  sublobular 
veins,  and  carry  the  blood  to  the  ascending  vena  cava  ;  their  walls 
are  thin  and  adherent  to  the  substance  of  the  hepatic  tissue. 

The  hepatic  ducts  or  bile  capillaries  originate  within  the  lobules, 
in  a  very  fine  plexus  lying  between  the  hepatic  cells ;  whether  the 
smallest  vessels  have  distinct  membranous  walls,  or  whether  they 
originate  in  the  spaces  between  the  cells  by  open  orifices,  has  not  been 
satisfactorily  determined. 


174  HUMAN   PHYSIOLOGY. 

The  bile-channels  empty  into  the  interlobular  ducts,  which  meas- 
ure about  ^olT  of  an  inch  in  diameter  and  are  composed  of  a  thin, 
homogeneous  membrane  lined  by  flattened  epithelial  cells. 

As  the  interlobular  bile-ducts  unite  to  form  large  trunks,  they 
receive  an  external  coat  of  fibrous  tissue,  which  strengthens  their 
walls  ;  they  finally  unite  to  form  one  large  duct  (the  hepatic  duct), 
which  joins  the  cystic  duct;  the  union  of  the  two  forms  the  ductus 
communis  choledochus,  which  is  about  three  inches  in  length,  the 
size  of  a  goose-quill,  and  opens  into  the  duodenum. 

The  gall-bladder  is  a  pear-shaped  sac,  about  four  inches  in  length, 
situated  in  a  fossa  on  the  under  surface  of  the  liver.  It  is  a  reser- 
voir for  the  bile,  and  is  capable  of  holding  about  one  ounce  and  a 
half  of  fluid.  It  is  composed  of  three  coats  : 

1.  Serous,  a  reflection  of  the  peritoneum. 

2.  Fibrous  and  muscular. 

3.  Mucous. 

Functions  of  the  Liver. — The  liver  is  a  complex  organ  having  a 
variety  of  relations  to  the  general  processes  of  the  body.  While 
its  physiologic  actions  are  not  yet  wholly  understood,  it  may  be  said 
that  it — 

1.  Secretes  bile. 

2.  Forms  glycogen. 

3.  'Assists  in  the  formation  of  urea  and  allied  products. 

The  Secretion  of  Bile. — The  characteristic  constituents  of  the 
bile  do  not  preexist  in  the  blood,  but  are  formed  in  the  interior 
of  the  liver  cells  of  materials  derived  from  the  venous  and  arterial 
blood.  The  hepatic  cells,  absorbing  these  materials,  elaborate  them 
into  bile-elements,  and  in  so  doing  undergo  histologic  changes  similar 
to  those  exhibited  by  other  secretory  glands.  The  bile  once  formed, 
it  passes  into  the  mouths  of  the  bile  capillaries,  near  the  periphery 
of  the  lobules.  Under  the  influence  of  the  vis  a  tergo  of  the  new- 
formed  bile  it  flows  from  the  smaller  into  the  large  bile-ducts,  and 
finally  empties  into  the  intestine,  or  is  regurgitated  into  the  gall- 
bladder, where  it  is  stored  up  until  it  is  required  for  the  digestive 
process  in  the  small  intestine.  The  study  of  the  secretion  of  bile 
by  means  of  biliary  fistulae  reveals  the  fact  that  the  secretion  is 
continuous  and  not  intermittent ;  that  the  hepatic  cells  are  constantly 
pouring  bile  into  the  ducts,  which  convey  it  into  the  gall-bladder. 
As  this  fluid  is  required  only  during  intestinal  digestion,  it  is  only 


LIVER.  175 

then    that   the    walls    of   the   gall-bladder    contract    and    discharge    it 
into  the  intestine. 

The  flow  of  bile  from  the  liver  cells  into  the  gall-bladder  is 
accomplished  by  the  inspiratory  movements  of  the  diaphragm,  and 
by  the  contraction  of  the  muscle-fibers  of  the  biliary  ducts,  as  well 
as  the  vis  a  tergo  of  new-formed  bile.  Any  obstacle  to  the  outflow 
of  bile  into  the  intestine  leaps  to  an  accumulation  within  the  bile- 
ducts.  The  pressure  within  the  ducts  increasing  beyond  that  of  the 
blood  within  the  capillaries,  a  reabsorption  of  biliary  matters  by 
the  lymphatics  takes  place,  giving  rise  to  the  phenomena  of  jaundice. 

The  bile  is  both  a  secretion  and  an  excretion;  it  contains  new  con- 
stituents, which  are  formed  only  in  the  substance  of  the  liver,  and 
are  destined  to  play  an  important  part  ultimately  in  nutrition ;  it 
contains  also  waste  ingredients,  which  are  discharged  into  the 
intestinal  canal  and  eliminated  from  the  body. 

Glycogenic  Function. — In  addition  to  the  preceding  function,  Ber- 
nard, in  1848,  demonstrated  the  fact  that  the  liver,  during  life, 
normally  produces  a  sugar-forming  substance,  analogous  in  its  chemic 
composition  to  starch,  which  he  terms  glycogen ;  also  that,  when  the 
liver  is  removed  from  the  body,  and  its  blood-vessels  are  thoroughly 
washed  out,  after  a  few  hours  sugar  again  makes  its  appearance  in 
abundance. 

It  can  be  shown  to  exist  in  the  blood  of  the  hepatic  vein  as  well 
as  in  a  decoction  of  the  liver  substance  by  means  of  either  Trom- 
mer's  or  Fehling's  test,  even  when  the  blood  of  the  portal  vein  does 
not  contain  a  trace  of  sugar. 

Origin  and  Destination  of  Glycogen. — Glycogen  appears  to  be 
formed  de  novo  in  the  liver  cells,  from  materials  derived  from  the 
food,  whether  the  diet  be  animal  or  vegetable,  though  a  larger  per- 
centage is  formed  when  the  animal  is  fed  on  starchy  and  saccharin 
than  when  fed  on  animal  food.  The  dextrose,  which  is  one  of  the 
products  of  digestion,  is  absorbed  by  the  blood-vessels  and  carried 
directly  into  the  liver;  as  it  does  not  appear  in  the  urine,  as  it 
would  if  injected  at  once  into  the  general  circulation,  it  is  probable 
that  it  is  detained  in  the  liver,  dehydrated,  and  stored  up  as  glycogen. 
The  change  is  shown  by  the  following  formula : 

C6H1206  -  H20  =  C6H1005. 
Dextrose.    Water.      Glycogen. 


176  HUMAN    PHYSIOLOGY. 

The  glycogen  thus  formed  is  stored  up  in  the  hepatic  cells  for  the 
future  requirements  of  the  system.  When  it  is  carried  from  the 
liver,  it  is  again  transformed  into  dextrose  by  the  agency  of  a  fer- 
ment. Glycogen  does  not  undergo  oxidation  in  the  blood ;  this 
process  takes  place  in  the  tissues,  particularly  in  the  muscles,  where 
it  generates  heat  and  contributes  to  the  development  of  muscular 
force. 

Glycogen,  when  obtained  from  the  liver,  is  an  amorphous,  starch- 
like  substance,  of  a  white  color,  tasteless  and  colorless,  and  soluble 
in  water  ;  by  boiling  with  dilute  acids,  or  subjected  to  the  action  of 
an  animal  ferment,  it  is  easily  converted  into  dextrose.  When  an 
excess  of  sugar  is  generated  by  the  liver,  dextrose  can  be  found 
not  only  in  the  blood  of  the  hepatic  vein,  but  also  in  other  portions 
of  the  body ;  under  these  circumstances  it  is  eliminated  by  the 
kidneys,  appearing  in  the  urine,  constituting  the  condition  of  gly- 
cosuria. 

Formation  of  Urea. — The  liver  is  now  regarded  by  many  physiolo- 
gists as  the  principal  organ  concerned  in  urea  formation.  The  liver 
normally  contains  a  certain  amount  of  urea ;  and  if  blood  be  passed 
through  the  excised  liver  of  an  animal  which  has  been  in  full  di- 
gestion when  killed,  a  large  amount  of  urea  i  obtained.  The 
clinical  evidence  proves  that  in  destructive  diseases  of  the  liver 
substance  there  is  at  once  a  falling-off  in  urea  elimination.  Various 
drugs  which  stimulate  liver  action  increase  the  amount  of  urea  in 
the  urine. 

Influence  of  the  Nerve  System. — The  nervous  system  directly 
controls  the  functional  activity  of  the  liver,  and  more  especially  its 
glycogenic  function.  It  was  discovered  by  Bernard  that  puncture 
of  the  medulla  oblongata  is  followed  by  so  enormous  a  production 
of  sugar  that  it  is  at  once  excreted  by  the  kidneys,  giving  rise  to 
diabetic  or  saccharine  urine.  This  part  of  the  medulla  is,  however, 
the  vaso-motor  center  for  the  blood-vessels  of  the  liver.  Destruction 
of  this  center,  or  injury  to  the  vaso-motor  nerves  emanating  from 
it  in  any  part  of  their  course,  is  followed  at  once  by  dilatation  of  the 
hepatic  blood-vessels,  slowing  of  the  blood-current,  a  profound  dis- 
turbance of  the  normal  relation  existing  between  the  blood  and 
liver-cells,  and  a  production  of  sugar.  Many  of  the  hepatic  vaso- 
motor  nerves  may  be  traced  down  the  cord  as  far  as  the  lumbar 
region,  while  others  leave  the  cord  high  up  in  the  neck  and  enter 


SKIN.  177 

the  cervical  ganglia  of  the  sympathetic,  and  so  reach  the  liver. 
Injury  to  the  sympathetic  ganglia  is  often  followed  by  diabetes. 
Peripheral  stimulation  of  various  nerves — e.  g.,  sciatic,  pneumogas- 
tric,  depressor  nerve, — as  well  as  the  direct  action  of  many  drugs, 
impair  or  depress  the  hepatic  vaso-motor  center  and  so  give  rise  to 
diabetes. 

SKIN. 

The  skin,  the  external  investment  of  the  body,  is  a  most  complex 
and  important  structure,  serving — • 

1.  As  a  protective  covering. 

2.  As  an  organ  for  tactile  sensibility. 

3.  As  an  organ  for  the  elimination  of  excrementitious  matters. 

The  amount  of  skin  investing  the  body  of  a  man  of  average  size 
is  about  twenty  feet,  and  varies  in  thickness,  in  different  situations, 
from  1  to  yip-  of  an  inch. 

The  skin  consists  of  two  principal  layers — viz.,  a  deeper  portion, 
the  corium,  and  a  superficial  portion,  the  epidermis. 

The  corium,  or  cutis  vera,  may  be  subdivided  into  a  reticulated 
and  a  papillary  layer.  The  former  is  composed  of  white  fibrous 
tissue,  non-striated  muscle-fibers,  and  elastic  tissue,  interwoven  in 
every  direction,  forming  an  areolar  network,  in  the  meshes  of  which 
are  deposited  masses  of  fat,  and  a  structureless,  amorphous  matter ; 
the  latter  is  formed  mainly  of  club-shaped  elevations  or  projections 
of  the  amorphous  matter,  constituting  the  papilla;  they  are  most 
abundant  and  well  developed  upon  the  palms  of  the  hands  and  upon 
the  soles  of  the  feet ;  they  average  y1^  of  an  inch  in  length,  and 
may  be  simple  or  compound ;  they  are  well  supplied  with  nerves, 
blood-vessels,  and  lymphatics. 

The  epidermis,  or  scarf  skin,  is  an  extravascular  structure,  a 
product  of  the  true  skin,  and  is  composed  of  several  layers  of  cells. 
It  may  be  divided  into  two  layers  :  the  rete  mucosum,  or  the  Mal- 
pighian  layer,  and  the  horny  or  corneous. 

The  former  is  closely  adherent  to  the  papillary  layer  of  the  true 
skin,  and  is  composed  of  large,  nucleated  cells,  the  lowest  layer  of 
which,  the  "  prickle  cells,"  contains  pigment-granules,  which  give  to 
the  skin  its  varying  tints  in  different  individuals  and  in  different 
races  of  men ;  the  more  superficial  cells  are  large,  colorless,  and 
semi-transparent.  The  latter,  the  corneous  layer,  is  composed  of 
13 


178  HUMAN   PHYSIOLOGY. 

flattened  cells,  which,  from  their  exposure  to  the  atmosphere,  are  hard 
and  horny  in  texture ;  it  varies  in  thickness  from  J-  of  an  inch  on  the 
palms  of  the  hands  and  soles  of  the  feet  to  ?Jg-  of  an  inch  in  the 
external  auditory  canal. 

APPENDAGES    OF   THE   SKIN. 

Hairs  are  found  in  almost  all  portions  of  the  body,  and  can  be 
divided  into — 

1.  Long,  soft  hairs,  on  the  head. 

2.  Short,  stiff  hairs,  along  the  edges  of  the  eyelids  and  nostrils. 

3.  Soft,  downy  hairs  on  the  general  cutaneous  surface. 

They  consist  of  a  root  and  a  shaft.  The  latter  is  oval  in  shape 
and  about  ¥^Q  of  an  inch  in  diameter ;  it  consists  of  fibrous  tissue, 
covered  externally  by  a  layer  of  imbricated  cells,  and  internally  by 
cells  containing  granular  and  pigment  material. 

The  root  of  the  hair  is  embedded  in  the  hair-follicle,  formed  by  a 
tubular  depression  of  the  skin,  extending  nearly  through  to  the 
subcutaneous  tissue ;  its  walls  are  formed  by  the  layers  of  the  corium, 
covered  by  epidermic  cells.  At  the  bottom  of  the  follicle  is  a 
papillary  projection  of  amorphous  matter,  corresponding  to  a  papilla 
of  the  true  skin,  containing  blood-vessels  and  nerves,  upon  which  the 
hair-root  rests.  The  investments  of  the  hair-roots  are  formed  of 
epithelial  cells,  constituting  the  internal  and  external  root-sheaths. 

The  hair  protects  the  head  from  the  heat  of  the  sun  and  from  the 
cold,  retains  the  heat  of  the  body,  prevents  the  entrance  of  foreign 
matter  into  the  lungs,  nose,  ears,  etc.  The  color  is  due  to  pigment 
matter.  In  old  age  the  hair  becomes  more  or  less  whitened. 

The  sebaceous  glands,  embedded  in  the  true  skin,  are  simple 
and  compound  racemose  glands,  opening,  by  a  common  excretory 
duct,  upon  the  surface  of  the  epidermis  or  into  the  hair-follicle. 
They  are  found  in  all  portions  of  the  body,  most  abundantly  in  the 
face,  and  are  formed  by  a  delicate,  structureless  membrane,  lined 
by  flattened  polyhedral  cells.  The  sebaceous  glands  secrete  a  pe- 
culiar oily  matter  (the  sebum},  by  which  the  skin  is  lubricated  and 
the  hairs  are  softened ;  it  is  quite  abundant  in  the  region  of  the  nose 
and  forehead,  which  often  presents  a  greasy,  glistening  appearance ; 
it  consists  of  water,  mineral  salts,  fatty  globules,  and  epithelial  cells. 

The  vernix  caseosa,  which  frequently  covers  the  surface  of  the 
fetus  at  birth,  consists  of  the  residue  of  the  sebaceous  matter,  con- 


SKIN.  179 

taining  epithelial  cells  and  fatty  matters ;  it  seems  to  keep  the  skin 
soft  and  supple,  and  guards  it  from  the  effects  of  the  long-continued 
action  of  the  amniotic  water. 

The  sudoriparous  glands  excrete  the  sweat.  They  consist  of  a 
mass  or  coil  of  a  tubular  gland  duct,  situated  in  the  derma  and  in  the 
subcutaneous  tissue,  average  ^  of  an  inch  in  diameter,  and  are 
surrounded  by  a  rich  plexus  of  capillary  blood-vessels.  From  this 
coil  the  duct  passes  in  a  straight  direction  up  through  the  skin  to 
the  epidermis,  where  it  makes  a  few  spiral  turns  and  opens  obliquely 
upon  the  surface.  The  sweat-glands  consist  of  a  delicate  homo- 
geneous membrane  lined  by  epithelial  cells,  whose  function  is  to 
extract  from  the  blood  the  elements  existing  in  the  perspiration. 

The  glands  are  very  abundant  all  over  the  cutaneous  surface — as 
many  as  3528  to  the  square  inch,  according  to  Erasmus  Wilson. 

The  perspiration  is  an  excrementitious  fluid,  clear,  colorless,  almost 
odorless,  slightly  acid  in  reaction,  with  a  specific  gravity  of  1003  to 
1004. 

The  total  quantity  of  perspiration  excreted  daily  has  been  esti- 
mated at  about  two  pounds,  though  the  amount  varies  with  the  nature 
of  the  food  and  drink,  exercise,  external  temperature,  season,  etc. 

The  elimination  of  the  sweat  is  not  intermittent,  but  continuous ; 
it  takes  place  so  gradually  that  as  fast  as  it  is  formed  it  passes  off 
by  evaporation  as  insensible  perspiration.  Under  exposure  to  great 
heat  and  exercise  the  evaporation  is  not  sufficiently  r.apid,  and  it 
appears  as  sensible  perspiration. 

COMPOSITION    OF    SWEAT. 

Water .     .  995.573 

Urea 0.043 

Fatty   matters 0.014 

Alkaline  lactates 0.317 

Alkaline  sudorates .  1.562 

Inorganic    salts      .     .     . 2.491 


1,000.000 

Urea  is  a  constant  ingredient. 

Carbonic   acid   is   also    exhaled    from    the   skin,    the    amount   being 
about    Z^Q  of  that  from  the  lungs. 

Perspiration  regulates  the  temperature  and  removes  waste  matters 


180  HUMAN   PHYSIOLOGY. 

from  the  blood ;  it  is  so  important  that  if  elimination  be  prevented, 
death  occurs  in  a  short  time. 

Influence  of  the  Nerve  System. — The  secretion  of  sweat  is  regu- 
lated by  the  nerve  system.  Here,  as  in  the  secreting  glands,  the 
fluid  is  formed  from  material  in  the  lymph-spaces  surrounding  the 
gland.  Two  sets  of  nerves  are  concerned — viz.,  vasomotor,  regulating 
the  blood-supply ;  and  secretor,  stimulating  the  activities  of  the  gland 
cells.  Generally  the  two  conditions,  increased  blood  flow  and  in- 
creased glandular  action,  coexist.  At  times  profuse  clammy  perspira- 
tion occurs,  with  diminished  blood  flow. 

The  dominating  sweat-center  is  located  in  the  medulla,  though 
subordinate  centers  are  present  in  the  cord.  The  secretory  fibers 
reach  the  perspiratory  glands  of  the  head  and  face  through  the 
cervical  sympathetic  ;  of  the  arms,  through  the  thoracic  sympathetic, 
ulnar,  and  radial  nerves  ;  of  the  leg,  through  the  abdominal  sympa- 
thetic and  sciatic  nerves. 

The  sweat-center  is  excited  to  action  by  mental  emotions,  in- 
creased temperature  of  blood  circulating  in  the  medulla  and  cord,  in- 
creased venosity  of  blood,  many  drugs,  rise  of  external  temperature, 
exercise,  etc. 


THE  CENTRAL  ORGANS  OF  THE  NERVE 
SYSTEM  AND  THEIR  NERVES. 

The  central  organs  of  the  nerve  system  are  the  encephalon  and  the 
spinal  cord,  lodged  within  the  cavity  of  the  cranium  and  the  cavity  of 
the  spinal  column  respectively.  The  general  shape  of  these  two  portions 
of  the  nerve  system  correspond  with  that  of  the  cavities  in  which 
they  are  contained.  The  encephalon  is  broad  and  ovoid,  the  spinal 
cord  is  narrow  and  elongated. 

The  encephalon  is  subdivided  by  deep  fissures  into  four  distinct, 
though  closely  related  portions:  viz.,  (i)  the  cerebrum,  the  large 
ovoid  mass  occupying  the  entire  upper  part  of  the  cranial  cavity ; 
(2)  the  cerebellum,  the  wedge-shaped  portion  placed  beneath  the 
posterior  part  of  the  cerebrum  and  lodged  within  the  cerebellar 
fossae;  (3)  the  isthmus  of  the  encephalon,  the  more  or  less  pyra- 
midal-shaped portion  connecting  the  cerebrum  and  cerebellum  with 
each  other  and  both  with  (4)  the  medulla  oblongata. 


CENTRAL  ORGANS   OF   NERVE   SYSTEM.  181 

The  spinal  cord  is  narrow  and  cylindric  in  shape.  It  occupies 
the  spinal  canal  as  far  down  as  the  second  or  third  lumbar  vertebra. 

The  nerves  in  relation  with  the  central  organs  of  the  nerve  system 
are  the  encephalic  or  cranial  and  the  spinal  nerves. 

The  encephalic  nerves  are  twelve  in  number  on  each  side  of  the 
median  line.  Because  of  the  fact  that  they  pass  through  foramina  in 
the  walls  of  the  cranium  they  are  usually  termed  cranial  nerves. 

The  spinal  nerves  are  thirty-one  in  number  on  each  side  of  the 
cord. 

The  cranial  and  spinal  nerves  are  ultimately  distributed  to  all  the 
structures  of  the  body — e.  g.,  the  general  periphery,  and  for  this 
reason  they  are  collectively  known  as  the  peripheral  organs  of  the 
nerve  system. 

The  central  organs  of  the  nerve  system  are  supported  and  pro- 
tected by  three  membranes  named,  in  their  order  from  without  inward, 
as  the  dura  mater,  the  arachnoid  and  the  pia  mater. 

The  dura  mater,  the  outermost  of  the  three,  is  a  tough  mem- 
brane, composed  of  white  fibrous  tissue  arranged  in  bundles,  which 
interlace  in  every  direction.  In  the  cranial  cavity  it  lines  the  inner 
surface  of  the  bones,  and  is  attached  to  the  edge  of  the  foramen 
magnum ;  it  sends  processes  inward,  forming  the  falx  cerebri,  falx 
cerebelli,  and  tentorium  cerebelli,  supporting  and  protecting  parts  of 
the  brain.  In  the  spinal  canal  it  loosely  invests  the  cord,  and  is 
separated  from  the  walls  of  the  canal  by  areolar  tissue. 

The  arachnoid,  the  middle  membrane,  is  a  delicate  serous  structure 
which  envelops  the  brain  and  cord,  forming  the  visceral  layer,  and  is 
then  reflected  to  the  inner  surface  of  the  dura  mater,  forming  the 
parietal  layer.  Between  the  two  layers  there  is  a  small  quantity  of 
fluid  which  prevents  friction  by  lubricating  the  two  surfaces. 

The  pia  mater,  the  most  internal  of  the  three,  composed  of 
areolar  tissue  and  blood-vessels,  covers  the  entire  surface  of  the 
brain  and  cord,  to  which  it  is  closely  adherent,  dipping  down  be- 
tween the  convolutions  and  fissures.  It  is  exceedingly  vascular, 
sending  small  blood-vessels  some  distance  into  the  brain  and  cord. 

The  cerebro-spinal  fluid  occupies  the  subarachnoid  space  and  the 
general  ventricular  cavities  of  the  brain,  which  communicate  by  an 
opening  (the  foramen  of  Magendie)  in  the  pia  mater,  at  the  lower 
portion  of  the  fourth  ventricle.  This  fluid  is  clear,  transparent,  alka- 


182  HUMAN   PHYSIOLOGY. 

line,  possesses  a  salty  taste  and  has  a  low  specific  gravity ;  it  is 
composed  largely  of  water,  traces  of  albumin,  glucose,  and  mineral 
salts.  The  quantity  is  estimated  from  two  to  four  fluidounces. 

The  function  of  the  cerebro-spinal  fluid  is  to  protect  the  brain 
and  cord  by  preventing  concussion  from  without;  by  being  easily 
displaced  into  the  spinal  canal,  prevents  undue  pressure  and  insuffi- 
ciency of  blood  to  the  brain. 


SPINAL    CORD. 

The  spinal  cord  varies  from  sixteen  to  eighteen  inches  in  length ; 
is  Y-Z  of  an  inch  in  thickness,  weighs  il/2  ounces,  and  extends  from 
the  atlas  to  the  second  lumbar  vertebra,  terminating  in  the  filum 
terminate.  It  is  cylindric  in  shape,  and  presents  an  enlargement  in 
the  lower  cervical  and  lower  dorsal  regions,  corresponding  to  the 
origin  of  the  nerves  which  are  distributed  to  the  upper  and  lower 
extremities.  The  cord  is  divided  into  two  lateral  halves  by  the 
anterior  and  posterior  fissures.  It  is  composed  of  both  zvhite  or 
fibrous  and  gray  or  vesicular  matter,  the  former  occupying  the  ex- 
terior of  the  cord,  the  latter  the  interior,  where  it  is  arranged  in 
the  form  of  two  crescents,  one  in  each  lateral  half,  united  by  the 
central  mass,  the  gray  commissure;  the  white  matter  being  united 
in  front  by  the  white  commissure. 

Structure  of  the  Gray  Matter. — The  gray  matter  is  arranged  in 
the  form  of  two  crescents,  united  by  a  commissural  band,  forming 
a  figure  resembling  the  letter  H.  Each  crescent  presents  an  anterior 
and  a  posterior  horn.  The  center  of  the  commissure  presents  a  canal 
which  extends  from  the  fourth  ventricle  downward  to  the  filum 
terminale.  The  anterior  horn  is  short  and  broad  and  does  not  extend 
to  the  surface.  The  posterior  horn  is  narrow  and  elongated  and 
extends  quite  to  the  surface.  It  is  covered  and  capped  by  the 
substantia  gelatinosa.  The  gray  matter  consists  primarily  of  a 
framework  of  fine  connective  tissue,  supporting  blood-vessels,  lym- 
phatics, medullated  and  non-medullated  nerve-fibers,  and  groups 
of  nerve-cells. 

The  nerve-cells  are  arranged  in  groups,  which  extend  for  some 
distance  throughout  the  cord,  forming  columns  more  or  less  continu- 
ous. The  first  group  is  situated  in  the  anterior  horn,  the  cells  of 
which  are  large,  multipolar,  and  connected  with  the  anterior  roots 


SPINAL  CORD. 


183 


a 


of  the  spinal  nerves,  and  are  supposed  to  be  motor  in  function.  The 
second  group  is  situated  in  the  posterior  horn,  the  cells  of  which  are 
spindle-shaped,  and  from  their  relation  to  the  posterior  roots  are 
supposed  to  be  sensory  in  function.  The  third  group  is  situated  in 
the  lateral  aspect  of  the  gray 
matter,  and  is  quite  separate 
and  distinct,  except  in  the 
lumbar  and  cervical  enlarge- 
ments, where  it  blends  with 
those  of  the  anterior  horn. 
A  fourth  group  is  situated 
at  the  inner  base  of  the  pos- 
terior horn ;  it  begins  about 
the  seventh  or  eighth  cervical 
nerve  and  extends  downward 
to  the  second  or  third  lum- 
bar, being  most  prominent 
in  the  dorsal  region.  This 
column  is  known  as  Clark's 
vesicular  column. 


hw 


FIG.  21. — SCHEME  OF  THE  CONDUCTING 
PATH  IN  THE  SPINAL  CORD  AT  THE 
THIRD  DORSAL  NERVE. — (Landois.) 

The  black  part  is  the  gray  matter,  v.  An- 
terior, hw,  posterior,  root.  a.  Direct, 
and  g,  g,  crossed,  pyramidal  tracts. 

b.  Anterior    column,    ground    bundle. 

c.  Coil's  column,     d.   Postero-external 
column,     e,  e,  and  f,  f.  Mixed  lateral 
p^ths.      h,   h.   Direct  cerebellar  tracts. 


Structure  of  the  White 
Matter. — The  white  matter 
surrounding  each  lateral  half 
of  the  cord  is  made  up  of 
nerve  fibers,  some  of  which  are  continuations  of  the  nerves  which 
enter  the  cord,  while  others  are  derived  from  different  sources.  It 
is  subdivided  into — 

1.  An  anterior  column,  comprising  that  portion  between  the  anterior 
roots  and  the  anterior  fissure,  which  is  again  subdivided  into  two 
parts : 

(a)  An  inner  portion,  bordering  the  anterior  median  fissure, 
the  direct  pyramidal  tract,  or  column  of  Tiirck ;  it  contains 
motor  fibers  which  do  not  decussate,  and  which  extend  as  far 
down  as  the  middle  of  the  dorsal  region. 

(fr)  An  outer  portion,  surrounding  the  anterior  cornua,  known 
as  the  anterior  root  zone,  composed  of  short,  longitudinal 
fibers  which  serve  to  connect  different  segments  of  the  spinal 
cord. 

2.  A  lateral  column,  the  portion  between  the  -anterior  and  posterior 
roots,  which  is  divisible  into — 


184  HUMAN   PHYSIOLOGY. 

(a)   The  crossed  pyramidal  tract,  occupying  the  posterior  portion 
of  the  lateral   column,   and  containing  all  those  fibers   of  the 
motor  tract  which  have  decussated  at  the  medulla  oblongata ; 
it  is  composed  of  longitudinally  running  fibers,  which  are  con- 
nected with  the  multipolar  nerve-cells  of  the  anterior  cornua. 
(fr)   The  direct  cerebellar  tract,  situated  upon  the  surface  of  the 
lateral  column,   consisting  of  longitudinal  fibers   which  termi- 
nate in  the  cerebellum ;   it  first  appears  in  the  lumbar  region, 
and  increases  in  thickness  as  it  passes  upward. 
(c)   The  anterior  tract,  lying  just  posterior  to  the  anterior  cornua. 
3.  A  posterior   column,   the   portion    included   between   the   posterior 
roots   and   the  posterior   fissure,    also   divisible   into   two   portions : 
(a)   An  inner  portion,  the  postero -internal  column,  or  the  column 

of  Goll,  bordering  the  posterior  median  fissure,  and 
(•&)   An  external  portion,  the  postero-external  column,  the  column 

of  Burdach,  lying  just  behind  the  posterior  roots. 
The  two  portions  of  the  posterior  column  are  composed  of  long  and 
short  commissural  fibers,  which  connect  different  segments  of  the 
spinal  cord. 

The  Relation  of  the  Spinal  Nerves  to  the  Spinal  Cord. — The 

spinal  nerves  present  near  the  spinal  cord  two  divisions  which  from 
their  connection  with  the  anterior  or  ventral  and  the  posterior  or 
dorsal  surfaces  are  known  as  the  anterior  or  ventral  and  posterior 
or  dorsal  roots.  The  ventral  roots  are  composed  of  nerve-fibers  which 
have  their  origin  in  the  nerve-cells  in  the  anterior  horns  of  the  gray 
matter.  The  dorsal  roots  are  composed  of  nerve-fibers  which  have 
their  origin  in  the  nerve-cells  in  the  spinal  ganglia.  After  entering. 
the  cord  some  of  the  posterior  or  dorsal  root-fibers  arborize  around 
nerve-cells  in  the  gray  matter  at  the  same  level ;  others  pass  obliquely 
upward  through  the  posterior  white  columns  as  far  as  the  nucleus 
gracilis  and  the  nucleus  cuneatus  around  the  nerve-cells  of  which 
they  terminate. 

FUNCTIONS    OF   THE    SPINAL    CORD. 

The  spinal  cord,  by  virtue  of  its  contained  nerve-cells  and  nerve- 
fibers,  may  be  regarded  as  composed  of — 

1.  Independent  nerve  centers  each  of  which  has  a  special  function; 
and — 

2.  Of    conducting    paths    by    which    these    centers    are    brought    into 


SPINAL   CORD.  185 

relation  with  one  another  and  with  the  cerebrum  and  its  subordi- 
nate or  underlying  parts. 

i.  As  an  Independent  Nerve  Center. 

The  spinal  cord,  by  virtue  of  its  contained  nerve-cells,  is  capable 
of  transforming  afferent  nerve  impulses  arriving  through  the  afferent 
nerves  into  efferent  impulses,  which  are  reflected  outward  through 
efferent  nerves  to  muscles,  producing  motion ;  to  glands,  exciting 
secretion ;  to  blood-vessels,  changing  their  caliber.  All  such  actions 
taking  place  independent  of  either  sensation  or  volition  are  termed 
reflex  actions.  The  mechanism  involved  in  every  reflex  action  con- 
sists of  a  sentient  surface,  an  afferent  nerve,  an  emissive  center, 
an  efferent  nerve,  and  a  responsive  organ,  muscle,  gland,  or  blood- 
vessels. 

The  reilex  excitability   of  the  cord   may  be — 

1.  Increased  by  disease  of  the  lateral  columns,  by  the  administration 
of  strychnin,  and,  in  frogs,  by  a  separation  of  cord  from  the  brain, 
the  latter  apparently  exerting  an  inhibitory  influence  over  the  for- 
mer and  depressing  its  reflex  activity. 

2.  Decreased    by    destructive    lesion    of    the    cord — e.    g.,    locomotor 
ataxia,    atrophy    of    the    anterior    cornua — the    administration    of 
various   drugs,   and,    in   the   frog,   by   irritation    of   certain   regions 
of  the  brain.     When  the  cerebrum  alone  is  removed  and  the  optic 
lobes  are  stimulated,  the  time  elapsing  between  the  application  of 
an  irritant  to  a  sensor  surface  and  the  resulting  movement  will  be 
considerably  prolonged,  the  optic  lobes    (Setschenow's   center)    ap- 
parently  generating    impulses    which,    descending   the    cord,    retard 
its  reflex  movement. 

Classification  of  Reflex  Movements  (Kiiss). — They  may  be  di- 
vided into  four  groups,  according  to  the  route  through  which  the 
afferent  and  efferent  impulses  pass  : 

1.  Those   normal   reflex   acts    (e.   g.,   deglutition,   coughing,   sneezing, 
walking,  etc.)   and  pathologic  reflex  acts   (e.  g.,  tetanus,  vomiting, 
epilepsy)    which  take  place  both  afferently  and  efferently  through 
spinal  nerves. 

2.  Reflex   acts   which   take  place   in   an   afferent   direction   through   a 
cerebro-spinal   sensor  nerve,   and   in  an   efferent   direction   through 
a   sympathetic   motor   nerve — e.   g.,   the   normal   reflex   acts,   which 
give  rise  to  most  of  the  secretions,  pallor  of  the  skin  and  blush- 
ing,   certain   movements    of   the    iris,    certain    modifications    in   the 


186  HUMAN   PHYSIOLOGY. 

beat  of  the  heart ;  the  pathologic  reflexes,  which,  on  account 
of  the  difficulty  in  explaining  their  production,  are  termed  meta- 
static — e.  g.,  ophthalmia,  coryza,  orchitis,  which  depend  on  a 
reflex  hyperemia;  amaurosis,  paralysis,  paraplegia,  etc.,  due  to  a 
reflex  anemia. 

3.  Reflex  movements   in   which  the   afferent  impulse  passes   through 
a    sympathetic    nerve,    and    the    efferent    through    a    cerebro-spinal 
nerve ;  most  of  these  phenomena  are  pathological — e.  g.,  convulsions 
from    intestinal    irritation    produced    by    the    presence    of    worms, 
eclampsia,  hysteria,  etc. 

4.  Reflex   actions   in   which   both   the   afferent  and   efferent   impulses 
pass  through  filaments   of  the   sympathetic  nervous   system — e.  g., 
those  obscure  reflex  actions  which  preside  over  the  secretions  of 
the  intestinal  fluids,  which  unite  the  phenomena  of  the  generative 
organs,    the    dilatation    of    the    pupils    from    intestinal    irritation 
(worms),  and  many  pathologic  phenomena. 

Laws  of  Reflex  Action  (Pfliiger). 

1.  Law  of  Unilaterality. — If  a  feeble  irritation  be  applied  to  one  or 
more  sensory  nerves,   movement  takes  place  usually   on   one   side 
only,  and  that  the  same  side  as  the  irritation. 

2.  Law    of    Symmetry. — If    the    irritation    becomes    sufficiently    in- 
tense, motor  reaction  is  manifested,  in  addition,   in  corresponding 
muscles  of  the  opposite  side  of  the  body. 

3.  Law  of  Intensity. — Reflex  movements  are  usually  more  intense  on 
the  side  of  the  irritation  ;  at  times  the  movements  of  the  opposite 
side  equal  them  in  intensity,  but  they  are  usually  less  pronounced. 

4.  Law  of  Radiation. — If  the   excitation   still   continues   to   increase, 
it  is  propagated  upward,   and  motor  reaction  takes  place  through 
centrifugal  nerves  coming  from  segments  of  the  cord  higher  up. 

5.  Law    of    Generalization. — When    the    irritation    becomes    very    in- 
tense,  it  is  propagated  in   the  medulla  oblongata ;   motor  reaction 
then  becomes  general,  and  it  is  propagated  up  and  down  the  cord, 
so  that  all   the   muscles   of  the  body  are  thrown   into   action,   the 
medulla    oblongata    acting    as    a    focus    whence    radiate    all    reflex 
movements. 

Special  Reflex  Movements. 

Among  the  reflexes  connected  with  the  more  superficial  portions 
of  the  body  there  are  some  which  are  so  frequently  either  exag- 
gerated or  diminished  in  pathologic  lesions  of  the  spinal  cord  that 


SPINAL  CORD.  187 

their  study  affords  valuable  indications  as  to  the  seat  and  character 
of  the  lesions.     They  may  be  divided  into — 

1.  Skin  or  superficial,  and 

2.  Tendon  or  deep  reflexes. 

The  skin  reflexes,  characterized  by  contraction  of  underlying 
muscles,  are  induced  by  irritation  of  the  skin — °e.  g.}  pricking,  pinch- 
ing, scratching,  etc.  The  following  are  the  principal  skin  reflexes : 

1.  Plantar   reflex,   consisting   of   contraction    of   the   muscles    of   the 
foot,   induced  by  stimulation  of  the  sole  of  the   foot ;   it  involves 
the  integrity  of  the  reflex  arc  through  the  lower  end  of  the  cord. 

2.  Gluteal    reflex,    consisting    of    contraction    of    the    glutei    muscles 
when    the    skin    over    the    buttock    is    stimulated ;    it    takes    place 
through  the  segments  giving  origin  to  the  fourth  and  fifth  lumbar 
nerves. 

3.  Cremasteric  reflex,  consisting  of   a   contraction   of   the   cremaster 
muscle  and  a  retraction  of  the  testicle  toward  the  abdominal  ring 
when  the  skin  on  the  inner  side  of  the  thigh  is  stimulated ;  it  de- 
pends upon  the  integrity  of  the  segments  giving  origin  to  the  first 
and  second  lumbar  nerves. 

4.  Abdominal   reflex,   consisting   of   a   contraction   of   the   abdominal 
muscles   when  the   skin   upon   the   side   of  the   abdomen   is   gently 
scratched;    its    production    requires    the    integrity    of    the    spinal 
segments  from  the  eighth  to  the  twelfth  dorsal  nerves. 

5.  Epigastric  reflex,   consisting  of  a   slight  muscular   contraction   in 
the  neighborhood   of  the   epigastrium   when   the   skin   between  the 
fourth  and  sixth  ribs  is  stimulated;  it  requires  the  integrity  of  the 
cord  between  the  fourth  and  seventh  dorsal  nerves. 

6.  The  scapular  reflex  consists  oi  a  contraction  of  the  scapular  muscles 
when  the  skin  between  the  scapulae  is  stimulated;  it  depends  upon 
the    integrity    of    the    cord    between    the   fifth    cervical    and    third 
dorsal  nerves. 

The  superficial  reflexes,  though  variable,  are  generally  present  in 
health.  They  are  increased  or  exaggerated  when  the  gray  matter 
pf  the  cord  is  abnormally  excited,  as  in  tetanus,  strychnia-poisoning, 
and  disease  of  the  lateral  columns. 

The  tendon  reflexes,  characterized  by  the  contraction  of  a  muscle, 
are   also    of   much    value   in   the   diagnosis    of   lesions    of   the   spinal 
cord  and  are  elicited  by  a  sharp  blow  on  a  tendon.     The  following 
are  the  principal  tendon  reflexes  : 
i.  Patellar  reflex,  or   knee-jerk,  consisting  of  a  contraction   of  the 


188  HUMAN   PHYSIOLOGY. 

extensor  muscles  of  the  thigh  when  the  ligamentum  patellae  is 
struck  between  the  patella  and  tibia.  This  reflex  is  best  observed 
when  the  legs  are  freely  hanging  over  the  edge  of  a  table.  The 
patellar  reflex  is  generally  present  in  health,  being  absent  in  only 
two  per  cent. ;  it  is  greatly  exaggerated  in  lateral  sclerosis  and  in 
descending  degeneration  of  the  cord ;  it  is  absent  in  locomotor 
ataxia  and  in  atrophic  lesions  of  the  anterior  gray  cornua. 

2.  Ankle-jerk  or  Ankle  Reflex. — If  the  extensor  muscles  of  the  leg 
be    placed    upon    the    stretch    and    the    tendo    Achillis    be    sharply 
struck,  a  quick  extension  of  the  foot  will  take  place. 

3.  Ankle-clonus. — This   consists   of  a  series   of  rhythmic  reflex   con- 
tractions of  the  gastrocnemius  muscle,  varying  in  frequency  from 
six  to  ten  a  second.     To  elicit  this  reflex,  pressure  is  made  upon 
the  sole  of  the  foot  so  as  suddenly  and  energetically  to  flex  the 
foot  at  the  ankle,  thus  putting  the  tendo  Achillis  and  the  gastroc- 
nemius   muscle    on    the    stretch.      The    rhythmic    movements    thus 
produced  continue  so  long  as  the  tension,   within  limits,   is  main- 
tained.    Ankle-clonus  is  never  present  in  health,  but  is  very  marked 
in  lateral  sclerosis  of  the  cord. 

The  toe  reflex,  per  one  al  reflex,  and  wrist  reflex  are  also  present 
in  sclerosis  of  the  lateral  columns  and  in  the  late  rigidity  of  hemi- 
plegia. 

Special  Nerve  Centers  in  Spinal  Cord. — Throughout  the  spinal 
cord  there  are  a  number  of  spinal  nerve  centers,  capable  of  being 
excited  reflexly  and  of  producing  complex  coordinated  movements. 
Though  for  the  most  part  independent  in  action,  they  are  subject  to 
the  controlling  influences  of  the  medulla  and  brain. 

1.  Ciliospinal  center,  situated  in  the  cord  between  the  lower  cervical 
and  the  third  dorsal  vertebra.     It  is  connected  with  the  dilatation 
of  the  pupil  through  fibers  which  emerge  in  this  region  and  enter 
the  cervical  sympathetic.     Stimulation  of  the  cord  in  this  locality 
causes  dilatation  of  the  pupil  on  the  same  side ;  destruction  of  the 
cord  is  followed  by  contraction  of  the  pupil. 

2.  Genitospinal  center,  situated  in  the  lower  part  of  the  cord.     This 
is   a   complex  center,   and   comprises   a   series   of   subordinate   cen- 
ters  for   the   control   of   the   muscular   movements   involved   in   the 
acts   of  defecation,   micturition,   and  ejaculation  of  semen,   and   of 
the  movements  of  the  uterus  during  parturition,  etc. 


SPINAL  CORD.  189 

3.  Vaso-motor    centers,    giving    origin    to    both    vaso-constrictor    and 
vaso-dilator    fibers,    which    are    distributed    throughout    the    cord. 
Though   acting   reflexly,   they   are   under   the   dominating   influence 

of  the  center  in  the  medulla. 

4.  Sweat -centers  are  also  present  in  various  parts  of  the  cord. 

2.  As  a  Conductor. 

The  white  matter  of  the  spinal  cord  consists  of  nerve-fibers,  the 
specific  function  of  which  is, 

1.  To    conduct    nerve    impulses    from    one    segment    of    the    cord    to 
another. 

2.  To    conduct   nerve   impulses   coming   from   the   encephalon    to    the 
spinal  cord  segments. 

3    To   conduct  nerve   impulses   coming  to   the   cord   through   afferent 
nerves,  directly  or  indirectly  to  the  encephalon. 

Intersegmental  Conduction. — The  spinal  cord  consists  of  a  series 
of  physiologic  segments  each  of  which  has  a  special  function  and  is 
associated  through  its  related  spinal  nerve  with  a  definite  segment 
of  the  body.  For  the  harmonious  cooperation  and  coordination  of 
all  the  spinal  segments  it  is  ^ssential  that  they  should  be  united 
by  commissural  or  associative  fibers.  The  cord  thus  becomes  capable 
of  complex  and  purposive  reflex  actions. 

Encephalo-spinal,  or  Motor  Conduction. — The  nerve-fibers  which 
conduct  volitional  impulses  from  the  brain  downward  to  the  an- 
terior cornua  arise  in  the  motor  centers  of  the  cerebrum ;  they 
then  pass  downward  through  the  corona  radiata,  the  internal  cap- 
sule, the  inferior  portions  of  the  crura  cerebri,  the  pons  Varolii, 
to  the  medulla  oblongata,  where  the  motor  tract  of  each  side  di- 
vides into  two  portions,  viz. : 

1.  The  larger,  containing  ninety-one  to  ninety-seven  per  cent,  of  the 
fibers,   which   decussates   at  the   lower  border   of  the  medulla   and 
passes  down  in  the  lateral  column  of  the  opposite  side,   and  con- 
stitutes the  crossed  pyramidal  tract. 

2.  The    smaller,    containing    three    to    nine    per    cent,    of    the    fibers, 
does  not  at  once  decussate,  but  passes  down  the  anterior  column 
of  the  same  side,  and  constitutes  the  direct  pyramidal  tract,  or  the 
column  of  Tiirck.     At  a  lower  level  this  tract  also   decussates  or 
crosses  over  to  the  opposite  side  of  the  cord. 

The  fibers  of  both  the  crossed  and  the  direct  pyramidal  tracts  come 
into  relation  by  their  terminal  branches  with  the  nerve-cells  in  the 


FIG.  22. — DIAGRAM  SHOWING  THE  COURSE,  THROUGH  THE  SPINAL  CORD,  OF  THE 
MOTOR  AND  SENSORY  NERVE-FIBERS. 

B  and  B'  represent  the  right  and  left  hemispheres  of  the  brain,  from  which 
the  motor  fibers  take  their  origin,  and  in  which  the  sensory  fibers  termi- 
nate. The  motor  tract  from  the  right  side,  i,  passes  down  through  the 
crus,  through  the  pons  to  the  medulla  oblongata,  where  it  divides  into  two 
portions:  (i)  The  larger  portion,  ninety-seven  per  cent.,  crosses  over  to 
the  opposite  side  of  the  cord  and  passes  down  through  the  lateral  column. 
It  gives  off  fibers  at  different  levels,  which  pass  into  the  gray  matter  and 
become  connected  with  the  muscles,  M,  through  the  multipolar  cells.  (2) 
The  smaller  portion,  three  per  cent.,  does  not  cross  over,  but  descends  on 
the  same  side  of  the  cord  in  the  anterior  column  and  supplies  the 
muscles,  m.  The  same  is  true  for  the  motor  tract  for  the  left  hemisphere. 

The  sensory  -fibers  from  the  left  side  of  the  body  enter  the  gray  matter  through 
the  posterior  roots.  They  then  cross  over  at  once  to  the  opposite  side  of 
the  cord  and  ascend  to  the  hemisphere,  partly  in  the  gray  matter,  partly 
in  the  posterior  column.  The  same  is  true  for  the  sensory  nerves  of 
the  right  side  of  the  body. 

190 


SPINAL  CORD.  191 

anterior  cornu  of  the  gray  matter  of  the  opposite  side  of  the 
cord.  (Fig.  22.) 

Through  this  decussation  each  half  of  the  cerebrum  governs  the 
muscle  movements  of  the  opposite  side  of  the  body. 

The  fibers  composing  the  crossed  and  the  direct  pyramidal  tracts 
are  therefore  the  channels  by  which  the  volitional  nerve  impulses 
are  conducted  from  the  motor  area  of  the  cortex  to  the  multipolar 
cells  in  the  anterior  cornua  of  the  gray  matter  of  the  spinal  cord, 
and  by  them  and  their  related  nerves  transmitted  t6  the  muscles. 

Spino-encephalic,  or  Sensor  Conduction. — The  nerve  impulses 
that  are  brought  to  the  spinal  cord  by  the  afferent  spinal  nerve- 
fibers  are  transmitted  by  afferent  paths  in  the  cord  for  the  most 
part  to  the  cortex  of  the  cerebrum  where  they  are  translated  into 
conscious  sensations.  These  paths  are  therefore  termed  sensor. 
The  sensor  tract  passes  through  the  cord,  the  medulla  oblongata,  the 
pons  Varolii,  the  superior  portion  of  the  crus  cerebri,  the  posterior 
third  of  the  posterior  limb  of  the  internal  capsule,  to  sensor  percep- 
tive areas  in  the  cerebral  cortex.  The  sensor  pathway  decussates 
at  all  levels  of  the  spinal  cord  and  medulla,  and  therefore  the  sensi- 
bility of  each  side  of  the  body  is  associated  with  the  opposite  side 
of  the  brain. 

The  paths  for  the  nerve  impulses  that  give  rise  to  different  sensa- 
tions have  been  variously  located  by  different  observers.  The  path- 
way for  the  impulses  that  give  rise  to  the  sensations  of  temperature 
has  been  located  in  the  gray  matter  ;  the  pathway  for  the  impulses 
that  give  rise  to  the  sensation  of  pain  has  been  located  in  Gower's 
tract ;  the  pathway  for  tactile  impressions  has  been  located  in  the 
posterior  columns. 

Properties  of  the  Spinal  Cord. — Irritation  applied  directly  to  the 
anterolateral  white  columns  produces  muscular  movements,  but  no 
pain ;  they  are,  therefore,  excitable,  but  insensible. 

The  surface  of  the  posterior  columns  is  not  sensitive  to  direct 
irritation,  except  near  the  origin  of  the  posterior  roots.  The  sensi- 
bility is  due,  however,  not  to  its  own  proper  fibers,  but  to  the  fibers 
of  the  posterior  root,  which  traverse  it. 

Division  of  the  anterolateral  columns  abolishes  all  power  of  vol- 
untary movement  in  the  lower  extremities. 

Division  of  the  posterior  column  impairs  the  power  of  muscular 
coordination,  such  as  is  witnessed  in  locomotor  ataxia. 


192  HUMAN    PHYSIOLOGY. 

The  gray  matter  is  probably  both  insensible  and  inexcitdble  under 
the  influence  of  direct  stimulation. 

A  transverse  section  of  one  lateral  half  of  the  cord  produces — 

1.  On   the   same   side,   paralysis   of   voluntary   motion,    a   relative   or 
absolute  elevation  of  temperature,  and  an  increased  flow  of  blood 
in   the   paralyzed   parts;    hyperesthesia,    for   the    sense   of   contact, 
tickling,  pain,  and  temperature. 

2.  On    the    opposite    side,    complete    anesthesia    as    regards    contact, 
tickling,    and    temperature    in    the    parts    corresponding    to    those 
which  are  paralyzed  in  the  opposite  side,  with  a  complete  preserva- 
tion of  voluntary  power  and  of  the  muscular  sense. 

A  vertical  section  through  the  middle  of  the.  gray  matter  results 
in  the  loss  of  sensation  on  both  sides  of  the  body  below  the  section, 
but  no  loss  of  voluntary  power. 

Paralysis  from  Injuries  of  the  Spinal  Cord. 

Seat  of  Lesion. — If  it  be  in  the  lower  part  of  the  sacral  canal, 
there  is  paralysis  of  the  compressor  urethrae,  accelerator  urinse,  and 
sphincter  ani  muscles  ;  no  paralysis  of  the  muscles  of  the  leg. 

At  the  Upper  Limit  of  the  Sacral  Region. — Paralysis  of  the  muscles 
of  the  bladder,  rectum  and  anus ;  loss  of  sensation  and  motion  in  the 
muscles  of  the  legs,  except  those  supplied  by  the  anterior  crural  and 
obturator — viz.,  psoas  iliacus,  sartorius,  pectineus,  adductor  longus, 
magnus,  and  brevis,  obturator,  vastus  externus  and  internus,  etc. 

At  the  Upper  Limit  of  the  Lumbar  Region. — Sensation  and  motion 
paralyzed  in  both  legs  ;  loss  of  power  over  the  rectum  and  bladder ; 
paralysis  of  the  muscular  walls  of  the  abdomen,  interfering  with 
expiratory  movements. 

At  the  Lower  Portion  of  the  Cervical  Region. — Paralysis  of  the 
legs,  etc.,  as  in  the  foregoing ;  in  addition,  paralysis  of  all  the  inter- 
costal muscles  and  consequent  interference  with  respiratory  move- 
ments ;  paralysis  of  muscles  of  the  upper  extremities,  except  those 
of  the  shoulders. 

Above  the  Middle  of  the  Cervical  Region. — In  addition  to  the 
preceding,  difficulty  of  deglutition  and  vocalization,  contraction  of  the 
pupils,  paralysis  of  the  diaphragm,  scalene  muscles,  intercostals, 
and  many  of  the  accessory  respiratory  muscles  ;  death  resulting  im- 
mediately from  arrest  of  respiratory  movements. 


THE    MEDULLA   OBLONGATA. 


193 


THE    MEDULLA    OBLONGATA. 

The  medulla  oblongata  is  the  expanded  portion  of  the  upper  part 
of  the  spinal  cord.  It  is  pyramidal  in  form  and  measures  il/2  inches 
in  length,  ^  of  an  inch  in  breadth,  y2  of  an  inch  in  thickness,  and 
is  divided  into  two  lateral  halves  by  the  anterior  and  posterior  median 
fissures,  which  are  continuous  with  those  of  the  cord.  Each  half 
is  again  subdivided  by  minor  grooves  into  four  columns — viz.,  an- 
terior, pyramid,  lateral  and  tract  olivary  body,  restiform  body,  and 
posterior  pyramid. 
.1.  The  anterior  pyramid  is  composed  partly  of  fibers  continuous 

with  those  of  the  anterior  column  of  the  spinal  cord,  but  mainly  of 


FIG.  23. — VIEW  OF  CEREBELLUM  IN  SECTION,  AND  OF  FOURTH  VENTRICLE,  WITH 
THE  NEIGHBORING  PARTS. — {From  Sappey.) 

i.  Median  groove  fourth  ventricle,  ending  below  in  the  calamus  scriptorius, 
with  the  longitudinal  eminences  formed  by  the  fasciculi  teretes,  one  on 
each  side.  2.  The  same  groove,  at  the  place  where  the  white  streaks  of 
the  auditory  nerve  emerge  from  it  to  cross  the  floor  of  the  ventricle.  3.  In- 
ferior peduncle  of  the  cerebellum,  formed  by  the  restiform  body.  4.  Pos- 
terior pyramid;  above  this  is  the  calamus  scriptorius.  5,  5.  Superior 
peduncle  of  cerebellum,  or  processes  e  ccrebello  ad  testes.  6,  6.  Fillet 
to  the  side  of  the  crura  cerebri.  7,  7.  Lateral  grooves  of  the  crura  cerebri. 
8.  Corpora  quadrigemina.  {After  Hirschfeld  and  Leveille.) 

fibers    derived    from    the    lateral    tract    of    the    opposite    side    by 
decussation.     The  united  fibers  then  pass  upward  through  the  pons 
14 


194  HUMAN    PHYSIOLOGY. 

Varolii  and  crura  cerebri,  and  for  the  most  part  terminate  in  the 
corpus  striatum  and  cerebrum. 

2.  The   lateral   tract   is   continuous   with   the  lateral   columns   of   the 
cord ;    its    fibers    in    passing    upward    take    three    directions — viz., 
an  external  bundle  joins  the  restiform  body,   and  passes  into  the 
cerebellum ;    an    internal    bundle    decussates    at    the    median    line 
and  joins  the  opposite  anterior  pyramid;  a  middle  bundle  ascends 
beneath  the  olivary  body,  behind  the  pons,  to  the  cerebrum,  as  the 
fasciculus  teres.     The  olivary  body  of  each  side  is  an  oval  mass, 
situated  between   the   anterior  pyramid   and   restiform   body;    it   is 
composed   of  white   matter   externally  and  gray  matter   internally, 
forming  the  corpus  dentatum. 

3.  The    restiform    body,    continuous    with    the    posterior    column    of 
the   cord,    also    receives   fibers    from    the   lateral    column.      As    the 
restiform  bodies  pass  upward  they  diverge  and  form  a  space   (the 
fourth   ventricle),   the   floor   of  which   is   formed   by   gray   matter, 
and  then  turn  backward  and  enter  the  cerebellum. 

4.  The    posterior    pyramid    is    a    narrow    white    cord    bordering    the 
posterior   median   fissure ;    it   is    continued   upward,    in    connection 
with  the  fasciculus  teres,  to  the  cerebrum. 

The  gray  matter  of  the  medulla  is  continuous  with  that  of  the 
cord.  It  is  arranged  with  much  less  regularity,  becoming  blended 
with  the  white  matter  of  the  different  columns,  with  the  exception 
of  the  anterior.  By  the  separation  of  the  posterior  columns  the  trans- 
verse commissure  is  exposed,  forming  part  of  the  floor  of  the 
fourth  ventricle ;  special  collections  of  gray  matter  are  found  in  the 
posterior  portions  of  the  medulla,  connected  with  the  roots  of  origin 
of  different  cranial  nerves. 

Properties  and  Functions. — The  medulla  is  excitable  anteriorly, 
and  sensitive  posteriorly,  to  direct  irritation.  It  serves — 

1.  As  a  conductor  of  afferent   or  sensor  impulses  upward   from  the 
cord,  through  the  gray  matter  to  the  cerebrum. 

2.  As  a  conductor  of  efferent,  volitional  or  motor  impulses  from  the 
brain  to  the  spinal  cord  and  nerves,  through  its  anterior  pyramids. 

3.  As  a  conductor  of  coordinating  impulses  from  the  spinal  cord  to 
the   cerebellum,   through   the  restiform  bodies. 

As  an  Independent  Reflex  Center. — The  medulla  oblongata  con- 
tains special  collections  of  gray  matter,  constituting  independent 


THE    MEDULLA   OBLONGATA.  195 

nerve  centers  presiding  over  different  functions,  some  of  which  are 
as   follows — viz. : 

1.  A   center  which   controls   the   movements   of   mastication,  through 
afferent  and  efferent  nerves. 

2.  A  center   reflecting   impressions   which   influence   the   secretion   of 
saliva. 

3.  A    center   for   sucking,    mastication,    and    deglutition,   whence    are 
derived    motor    stimuli    exciting    to    action    and    coordinating    the 
muscles  of  the  palate,  pharynx,  and  esophagus,   necessary   for  the 
swallowing  of  the  food. 

4.  A  center  which   coordinates  the  muscles   concerned  in  the  act  of 
vomiting. 

5.  A  speech  center,  coordinating  the  various  muscles   necessary   for 
the  accomplishment  of  articulation  through  the  hypoglossal,  facial 
nerves,   and  the   second   division   of  the  fifth   pair. 

6.  A  center  for  the  harmonization  of  muscles  concerned  in  expression, 
reflecting  its  impulses  through  the  facial  nerve. 

7.  A  cardiac  center,  which  exerts   (i)   an  accelerating  influence  over 
the   heart's   pulsations    through    accelerating   nerve-fibers    emerging 
from  the  cervical  portion  of  the  cord,  entering  the  inferior  cervical 
ganglion,   and  thence  passing  to   the   heart ;    (2)    an   inhibitory   or 
retarding   influence   upon   the    action    of   the   heart,    through   fibers 
of  the  spinal  accessory  nerve  running  in  the  trunk  of  the  pneumo- 
gastric.     The  cardio-inhibitory  center  is  in  a  state  of  tonic  activity 
and  continuously  sends   impulses  to  the  heart  which  exert  an   in- 
hibitory influence  upon   its  action.      It  may  be  stimulated  directly 
by   anemia   as   well   as   by   venous   hyperemia   of   the   blood-vessels 
of  the  medulla  and  increased  venosity  of  the  blood.     It  is  excited 
reflexly  by  the  stimulation  of  the  central  end  of  the  vagus,  sciatic, 
and  splanchnic  nerves. 

8.  A  vaso-motor  center,  which,  by  alternately  contracting  and  dilating 
the  blood-vessels  through   nerves   distributed   in  their  walls,   regu- 
lates the  quantity  of  blood  distributed  to  an  organ  or  tissue,  and 
thus  influences  nutrition,   secretion,  and  calorification.      The  vaso- 
motor  center  is  situated  in  the  medulla  oblongata  and  pons  Varolii, 
between    the    corpora    quadrigemina    and    the    calamus    script  orius. 
The  vaso-motor  fibers   having  their  origin   in  this  center  descend 
through    the    interior    of    the    cord,    emerge    through    the    anterior 
roots  of  spinal  nerves,   enter  the   ganglia  of  the   sympathetic,   and 
thence   pass   to   the   walls    of   the   blood-vessels,    and   maintain   an 


196  HUMAN   PHYSIOLOGY. 

arterial  tonus ;  they  may  be  divided  into  two  classes — viz.,  vaso- 
dilators (e.  g.,  chorda  tympani)  and  vaso-constrictors  (e.  g., 
sympathetic  fibers). 

Division  of  the  cord  at  the  lower  border  of  the  medulla  is  fol- 
lowed by  a  dilatation  of  the  entire  vascular  system  and  a  marked 
fall  of  the  blood  pressure.  Electric  stimulation  of  the  distal  surface 
of  the  cord  is  followed  by  a  contraction  of  the  blood-vessels  and  a 
rise  in  the  blood  pressure. 

The  vaso-motor  center  is  stimulated  directly  by  the  condition  of 
the  blood  in  the  medulla  oblongata.  When  the  blood  is  highly  venous 
this  center  becomes  very  active,  the  blood-vessels  throughout  the 
body  are  contracted,  and  the  blood  current  becomes  swifter ;  sudden 
anemia  of  the  medulla  has  a  similar  effect.  The  action  of  the  vaso- 
motor  center  may  be  accelerated,  with  attendant  rise  of  blood  pres- 
sure, by  irritation  of  certain  afferent  nerve-fibers.  These  are  known 
as  pressor  fibers.  On  the  other  hand,  its  action  may  be  depressed 
by  other  fibers,  with  attendant  fall  of  blood  pressure.  These  are 
known  as  depressor  fibers. 

9.  A   diabetic   center,  irritation   of  which   causes   an   increase  in   the 
amount   of   urine   secreted   and   the    appearance    of   a    considerable 
quantity  of  sugar  in  the  urine. 

10.  Respiratory  center,  situated  near  the  origin  of  the  pneumogastric 
nerves,  presides  over  the  movements  of  respiration  and  its  modifi- 
cations, laughing,  singing,  sobbing,  sneezing,  etc.     It  may  be  excited 
renexly  by  the  presence  of  carbonic  acid  in  the  lungs  irritating  the 
terminal  pneumogastric  filaments  ;  or  automatically,  according  to  the 
character   of  the  blood   circulating  through   it ;    an   excess   of  car- 
bonic  acid   or   a   diminution   of   oxygen   increasing   the   number   of 
respiratory  movements ;  a  reverse  condition  diminishing  the  respi- 
ratory movements. 

11.  A  spasm   center,   stimulation   of  which   gives   rise   to   convulsive 
phenomena,  such  as  coughing,  sneezing,  etc. 

12.  A  center  for  certain  ocular  functions,  governing  the  closure  of 
the  eyelids  and  dilatation  of  the  pupil. 

13.  A  sweat  center  is  also  localized  in  the  medulla. 


THE  CRURA   CEREBRI.  197 

THE   PONS    VAROLIL 

The  pons  Varolii  is  united  with  the  cerebrum  above,  the  cerebellum 
behind,  and  the  medulla  oblongata  below.  It  consists  of  transverse 
and  longitudinal  fibers,  amidst  which  are  irregularly  scattered  collec- 
tions of  gray  or  vesicular  nervous  matter. 

The  transverse  fibers  unite  the  two  lateral  halves  of  the  cerebellum. 

The   longitudinal   fibers   are   continuous — 

1.  With    the    anterior    pyramids    of    the    medulla    oblongata,    which, 
interlacing  with  the  deep  layers  of  the  transverse  fibers,  ascend  to 
the  crura  cerebri,  forming  their  superficial  or  fasciculated  portions. 

2.  With   fibers   derived   from   the   olivary   fasciculus,   some   of  which 
pass    to    the    tubercula    quadrigemina,    while    others,    uniting   with 
fibers    from    the    lateral    and    posterior    columns    of    the    medulla, 
ascend  in  the  deep  or  posterior  portions  of  the  crura  cerebri. 

Properties  and  Functions. — The  superficial  portion  is  insensible 
and  inexcitable  to  direct  irritation ;  the  deeper  portion  appears  to 
be  excitable,  consisting  of  descending  motor  fibers ;  the  posterior  por- 
tions are  sensible,  but  inexcitdble  to  irritation. 

Transmits  motor  and  sensor  impulses  from  and  to  the  cerebrum. 

The  gray  .ganglionic  matter  consists  of  centers  which  convert  im- 
pressions into  more  or  less  conscious  sensations  and  originate  motor 
impulses,  these  taking  place  independent  of  any  intellectual  process  ; 
they  are  the  seat  of  instinctive  reflex  acts,  the  centers  which  assist 
in  the  coordination  of  the  automatic  movements  of  station  and  pro- 
gression. 

THE    CRURA    CEREBRI. 

The  crura  cerebri  are  largely  composed  of  the  longitudinal  fibers 
of  the  pons  (anterior  pyramids,  fasciculi  teretes)  ;  after  emerging 
from  the  pons  they  increase  in  size,  and  become  separated  into  two 
portions  by  a  layer  of  dark-gray  matter,  the  locus  niger. 

The  superficial  portion,  the  crusta,  composed  of  the  anterior  pyra- 
mids, constitutes  the  motor  tract,  which  terminates,  for  the  most 
part,  in  the  corpus  striatum,  but  to  some  extent,  also,  in  the  cerebrum  ; 
the  deep  portion,  made  up  of  the  fasciculi  teretes  and  posterior 
pyramids  and  accessory  fibers  from  the  cerebellum,  constitutes  the 
sensor  tract  (the  tegmentum),  which  terminates  in  the  optic  thalamus 
and  cerebrum. 


198  HUMAN   PHYSIOLOGY. 

Function. — The  crura  are  conductors  of  motor  and  sensor  impulses  ; 
the  gray  matter  assists  in  the  coordination  of  the  complicated  move- 
ments of  the  eyeball  and  iris,  through  the  motor  oculi  communis 
nerve.  It  also  assists  in  the  harmonization  of  the  general  muscular 
movements,  as  section  of  one  crus  gives  rise  to  peculiar  movements 
of  rotation  and  somersaults  forward  and  backward. 


THE    CORPORA    QUADRIGEMINA. 

The  corpora  quadigemina  are  four  small,  rounded  eminences, 
two  on  each  side  of  the  median  line,  situated  immediately  behind 
the  third  ventricle,  and  beneath  the  posterior  border  of  the  corpus 
callosum. 

The  anterior  tubercles  are  oblong  from  before  backward,  and 
larger  than  the  posterior,  which  are  hemispheric  in  shape ;  they  are 
grayish  in  color,  but  consist  of  white  matter  externally  and  gray 
matter  internally. 

Both  the  anterior  and  posterior  tubercles  are  connected  with  the 
optic  thalami  by  commissural  bands,  named  the  anterior  and  pos- 
terior brachia,  respectively.  They  receive  fibers  from  the  olivary 
fasciculus  and  fibers  from  the  cerebellum,  which  press  upward  to 
enter  the  optic  thalami. 

The  corpora  geniculata  are  situated,  one  on  the  inner  side  and 
one  on  the  outer  side  of  each  optic  tract,  behind  and  beneath  the 
optic  thalamus,  and  from  their  position  are  named  the  corpora 
geniculata  internet  and  externa ;  they  give  origin  to  fibers  of  the 
optic  nerve. 

Functions. — The  corpora  quadrigemina  are  centers  associated  with 
the  visual  centers.  Destruction  of  these  tubercles  is  immediately 
followed  by  a  loss  of  the  sense  of  sight ;  moreover,  their  action  in 
vision  is  crossed,  owing  to  the  decussation  of  the  optic  tracts,  so  that 
if  the  tubercle  of  the  right  side  be  destroyed  by  disease  or  extir- 
pated, the  sight  is  lost  in  the  eye  of  the  opposite  side,  and  the  iris 
loses  its  mobility. 

The  tubercula  quadrigemina  as  nerve  centers  preside  over  the 
reflex  movements  which  cause  a  dilatation  or  contraction  of  the 
iris,  irritation  of  the  tubercles  causing  contraction,  destruction 
causing  dilatation.  Removal  of  the  tubercles  on  one  side  pro- 


CORPORA    STRIATA   AND   OPTIC   THALAMI.  199 

duces  a  temporary  loss  of  power  of  the  opposite  side  of  the  body, 
and  a  tendency  to  mo.ve  around  an  axis  is  manifested,  as  after 
a  section  of  one  crus  cerebri,  which,  however,  may  be  due  to  giddi- 
ness and  loss  of  sight. 

They  also  assist  in  the  coordination  of  the  complex  movements  of 
the  eye,  and  regulate  the  changes  of  the  iris  during  the  movements 
of  accommodation. 


CORPORA    STRIATA    AND    OPTIC    THALAMI. 

The  corpora  striata  are  two  large  ovoid  collections  of  gray 
matter,  situated  at  the  base  of  the  cerebrum,  the  larger  portions 
of  which  are  embedded  in  the  white  matter,  the  smaller  portions  pro- 
jecting into  the  anterior  part  of  the  lateral  ventricle.  Each  striated 
body  is  divided,  by  a  narrow  band  of  white  matter,  into  two  por- 
tions— viz. : 

1.  The  caudate  nucleus,  the  intraventricular  portion,  which  is  conic 
in   shape,    having   its   apex   directed   backward,    as   a   narrow,    tail- 
like  process. 

2.  The    lenticular  nucleus,   embedded   in    the   white   matter,    and    for 
the  most  part  external  to  the  ventricle.     On  the  outer  side  of  the 
lenticular   nucleus   is    found   a   narrow   band   of   white   matter,   the 
external    capsule;    and    between    it    and    the    convolutions    of    the 
island  of  Reil,  a  thin  band  of  gray  matter,  the  claustrum. 

The  corpora  striata  are  grayish  in  color,  and  when  divided,  present 
transverse  striations,  from  the  intermingling  of  white  fibers  and 
gray  cells. 

The  optic  thalami  are  two  oblong  masses  situated  in  the  ven- 
tricles posterior  to  the  corpora  striata,  and  resting  upon  the  posterior 
portion  of  the  crura  cerebri.  The  internal  surface,  projecting  into 
the  lateral  ventricles,  is  white,  but  the  interior  is  grayish,  from  a 
commingling  of  both  white  fibers  and  gray  cells.  Separating  the 
lenticular  nucleus  from  the  caudate  nucleus  and  the  optic  thalamus 
is  a  band  of  white  tissue,  the  internal  capsule. 

The  internal  capsule  is  a  narrow,  curved  tract  of  white  matter, 
and  is,  for  the  most  part,  an  expansion  of  the  motor  tract  of  the 
crura  cerebri.  It  consists  of  two  segments — an  anterior,  situated 
between  the  caudate  nucleus  and  the  anterior  surface  of  the  lenticular 
nucleus,  and  a  posterior,  situated  between  the  optic  thalamus  and  the 


200  HUMAN   PHYSIOLOGY. 

posterior  surface  of  the  lenticular  nucleus.  These  two  segments 
unite  at  an  obtuse  angle,  which  is  directed  toward  the  median  line. 
Pathologic  observation  has  shown  that  the  nerve-fibers  of  the  direct 
and  crossed  pyramidal  tracts  can  be  traced  upward  through  the 
anterior  two  thirds  of  the  posterior  segment  into  the  centrum  ovale, 
where,  for  the  most  part,  they  are  lost ;  a  portion,  however,  remaining 
united,  ascend  higher  and  terminate  in  the  paracentral  lobule,  and 
in  the  ascending  frontal  convolution.  Those  of  the  sensor  tract  can 
be  traced  upward,  through  the  posterior  third,  into  the  cerebrum, 
where  they  probably  terminate  in  the  ascending  parietal  and  the 
superior  parietal  convolutions  and  in  the  gyrus  fornicatus. 

Functions. — The  corpora  striata  are  the  centers  in  which  terminate 
some  of  the  fibers  of  the  superficial  or  motor  tract  of  the  crura 
cerebri ;  others  pass  upward  through  the  internal  capsule,  to  be  dis- 
tributed to  the  cerebrum.  It  might  be  inferred,  from  their  anatomic 
relations,  that  the  corpora  striata  are  motor  centers.  Irritation  by  a 
weak  galvanic  current  produces  muscular  movements  of  the  opposite 
side  of  the  body ;  destruction  of  their  substance  by  a  hemorrhage, 
as  in  apoplexy,  is  followed  by  a  paralysis  of  motion  of  the  opposite 
side  of  the  body,  but  there  is  no  loss  of  sensation.  When  the 
hemorrhagic  destruction  involves  the  fibers  of  the  anterior  two 
thirds  of  the  posterior  segment  of  the  internal  capsule,  and  thus 
separates  them  from  their  trophic  centers  in  the  cortical  motor 
region,  a  descending  degeneration  is  established,  which  involves 
the  direct  pyramidal  tract  of  the  same  side  and  the  crossed  pyra- 
midal tract  of  the  opposite  side. 

Destruction  of  the  posterior  one  third  of  the  posterior  segment 
of  the  internal  capsule  is  followed  by  a  loss  of  sensation  on  the 
opposite  side  of  the  body  and  a  loss  of  the  senses  of  smell  and  vision 
on  the  same  side  (Charcot).  The  precise  function  of  the  corpora 
striata  is  unknown,  but  they  are  in  some  way  connected  with  motion. 

The  optic  thalami  receive  the  fibers  of  the  tegmentus,  the  posterior 
portion  of  the  crura  cerebri.  They  are  insensible  and  inexcitable 
to  direct  irritation.  Removal  of  one  optic  thalamus,  or  destruction 
of  its  substance  by  disease  or  hemorrhage,  is  followed  by  a  loss 
of  sensibility  of  the  opposite  side  of  the  body,  but  there  is  no  loss 
of  motion ;  their  precise  function  is  also  unknown,  but  they  are  in 
some  way  connected  with  sensation.  In  both  cases  their  action  is 
crossed. 


THE   CEREBELLUM.  201 

THE    CEREBELLUM. 

The  cerebellum  is  situated  in  the  inferior  fossae  of  the  occipital 
bone,  beneath  the  posterior  lobes  of  the  cerebrum.  It  attains  its 
maximum  weight,  which  is  about  five  ounces,  between  the  twenty- 
fifth  and  fortieth  years,  the  proportion  between  the  cerebellum  and 
cerebrum  being  as  i  to  8^. 

It  is  composed  of  two  lateral  hemispheres  and  a  central  elongated 
lobe,  the  vermiform  process ;  the  two  hemispheres  are  connected  with 
each  other  by  the  fibers  of  the  middle  peduncle,  forming  the  super- 
ficial portion  of  the  pons  Varolii.  The  cerebellum  is  brought  into 
connection  with  the  medulla  oblongata  and  spinal  cord  through  the 
prolongation  of  the  restiform  bodies ;  with  the  cerebrum,  by  fibers 
passing  upward  beneath  the  corpora  quadrigemina  and  the  optic 
thalami,  and  then  forming  part  of  the  diverging  cerebral  fibers. 

Structure. — It  is  composed  of  both  white  and  gray  matter,  the 
former  being  internal,  the  latter  external,  and  is  convoluted,  for 
economy  of  space. 

The  white  matter  consists  of  a  central  stem,  the  interior  of  which 
is  a  dentated  capsule  of  gray  matter,  the  corpus  dentatum.  From 
the  external  surface  of  the  stem  of  white  matter  processes  are  given 
off,  forming  the  lamina,  which  are  covered  with  gray  matter. 

The  gray  matter  is  convoluted  and  covers  externally  the  laminated 
processes ;  a  vertical  section  through  the  gray  matter  reveals  the 
following  structures : 

1.  A   delicate    connective-tissue   layer,   just   beneath   the   pia   mater, 
containing  rounded  corpuscles,   and  with   branching  fibers  passing 
toward  the  external  surface. 

2.  The  cells  of  Purkinje,  forming  a  layer  of  large,  nucleated,  branched 
nerve-cells  sending  off  processes  to  the  external  layer. 

3.  A  granular  layer  of  small  but  numerous  corpuscles. 

4.  A  nerve-fiber  layer,  formed  by  a  portion  of  the  white  matter. 

Properties  and  Functions. — Irritation  of  the  cerebellum  is  not 
followed  by  any  evidences  either  of  pain  or  convulsive  movements ; 
it  is,  therefore,  insensible  and  inexcitable. 

Coordination  of  Movements. — Removal  of  the  superficial  portions 
of  the  cerebellum  in  pigeons  produces  feebleness  and  want  of  har- 
mony in  the  muscular  movements  ;  as  successive  slices  are  removed, 


202  HUMAN   PHYSIOLOGY. 

the  movements  become  more  irregular,  and  the  pigeon  becomes  rest- 
less ;  when  the  last  portions  are  removed,  all  power  of  Hying,  walking, 
standing,  etc.,  is  entirely  gone,  and  the  equilibrium  can  not  be  main- 
tained, the  power  of  coordinating  muscular  movements  being  wholly 
lost.  The  same  results  have  been  obtained  by  operating  on  all  classes 
of  animals. 

The  following  symptoms  were  noticed  by  Wagner,  after  removing 
the  whole  or  a  large  part  of  the  cerebellum : 

1.  A  tendency  on  the  part  of  the  animal  to  throw  itself  on  one  side, 
and  to  extend  the  legs  as  far  as  possible. 

2.  Torsion  of  the  head  on  the  neck. 

3.  Trembling  of  the  muscles  of  the  body,  which  was  general. 

4.  Vomiting  and  occasional  liquid  evacuations. 

Forced  Movements. — Division  of  one  crus  cerebelli  causes  the 
animal  to  fall  on  one  side  and  roll  rapidly  on  its  longitudinal  axis. 
According  to  Schiff,  if  the  peduncle  be  divided  from  behind,  the  ani- 
mal falls  on  the  same  side  as  the  injury ;  if  the  section  be  made  in 
front,  the  animal  turns  to  the  opposite  side. 

Disease  of  the  cerebellum  partially  corroborates  the  result  of 
experiments ;  in  many  cases  symptoms  of  unsteadiness  of  gait, 
from  a  want  of  coordination,  have  been  noticed. 

Comparative  anatomy  reveals  a  remarkable  correspondence  between 
the  development  of  the  cerebellum  and  the  increase  in  complexity  of 
muscular  actions.  It  attains  a  much  greater  development,  relatively 
to  the  rest  of  the  brain,  in  those  animals  whose  movements  are  very 
complex  and  varied  in  character,  such  as  the  kangaroo,  shark,  and 
swallow. 

The  cerebellum  may  possibly  exert  some  influence  over  the  sexual 
functions,  but  physiologic  and  pathologic  facts  are  opposed  to  the 
idea  of  its  being  the  seat  of  the  sexual  instinct.  It  appears  to  be 
simply  a  center  for  the  coordination  and  equilibration  of  muscular 
movements. 

THE    CEREBRUM. 

The  cerebrum  is  the  largest  portion  of  the  encephalic  mass,  con- 
stituting about  four  fifths  of  its  weight;  the  average  weight  of 
the  adult  male  brain  is  from  forty-eight  to  fifty  ounces,  or  about 
three  pounds  while  that  of  the  adult  female  is  about  five  ounces  less. 


THE   CEREBRUM.  203 

After  the  age  of  forty  the  weight  of  the  cerebrum  gradually  dimin- 
ishes at  the  rate  of  one  ounce  every  ten  years.  In  idiots  the  brain 
weight  is  often  below  the  normal,  at  times  not  amounting  to  more 
than  twenty  ounces. 

The  cerebrum  is  connected  with  the  pons  Varolii  and  medulla 
oblongata  through  the  crura  cerebri,  and  with  the  cerebellum  through 
the  superior  peduncles.  It  is  divided  into  two  lateral  halves,  or 
hemispheres,  by  the  longitudinal  fissure  running  from  before  back- 
ward in  the  median  line ;  each  hemisphere  is  composed  of  both  white 
and  gray  matter,  the  former  being  internal,  the  latter  external ;  it 
covers  the  surfaces  of  the  hemisphere  which  are  infolded,  forming 
fissures  and  convolutions. 

Fissures. 

1.  The  fissure  of  Sylvius  is  one  of  the  most  important;  it  is  the  first 
to  appear  in  the  development  of  the   fetal  brain,  being  visible  at 
about   the   third    month ;    in   the    adult    it    is    quite    deep    and   well 
marked,  running  from  the  under  surface  of  the  brain  upward,  out- 
ward,  and  backward,   and   forms   a   boundary   between   the   frontal 
and  temporosphenoid  lobes. 

2.  The  fissure  of  Rolando  is  second  in  importance,  and  runs  from  a 
point  on  the  convexity  near  the  median  line  transversely  outward 
and   downward  toward  the  fissure   of   Sylvius,   but  does   not  enter 
it.     It  separates  the  frontal  from  the  parietal  lobe. 

3.  The   parietal   fissure,   arising   a   short   distance   behind   the   fissure 
of  Rolando,  upon  the  convexity  of  the  hemisphere,  runs  downward 
and  backward  to  its  posterior  extremity. 

4.  The    parieto-occipital    fissure    separates    the    occipital     from    the 
parietal  lobe.     Beginning  upon  the  outer  surface  of  the  cerebrum, 
it  is  continued  on  the  mesial  aspect  downward  and  forward  until 
it  terminates  in  the  calcarine  fissure. 

5.  The  callosomarginal  fissure  lies  upon  the  mesial  surface,  where  it 
runs  parallel  with  the  corpus  callosum. 

Secondary  fissures  of  importance  are  found  in  different  lobes  of 
the  cerebrum,  separating  the  various  convolutions.  In  the  anterior 
lobe  are  found  the  precentral,  superior  frontal,  and  inferior  frontal 
fissures ;  in  the  temporosphenoid  lobes  are  found  the  first  and  second 
temporosphenoid  fissure;  in  the  occipital  lobe,  the  calcarine  and 
hippocampal  fissures. 


204* 


HUMAN   PHYSIOLOGY. 


Convolutions.    Frontal  Lobe. 

The  ascending  frontal  or  precentral  convolution,  situated  in  front 
of  the  fissure   of  Rolando   runs   downward   and   forward ;   it   is   con- 


FIG.  .24. — DIAGRAM  SHOWING  FISSURES  AND  CONVOLUTIONS  OF  THE  LEFT  SIDE 
OF  THE  HUMAN   BRAIN. —  (Landois'   "Physiology.") 

F.  Frontal.  P.  Parietal.  O.  Occipital.  T.  Temporosphenoid  lobe.  S.  Fis- 
sure of  Sylvius.  S'.  Horizontal.  S".  Ascending  ramus  of  S.  c.  Sulcus 
centralis,  or  fissure  of  Rolando.  A.  Ascending  frontal,  and  B.  Ascending 
parietal  convolutions.  FI.  Superior,  F2.  Middle,  and  F3.  Inferior  frontal 
convolutions.  f2.  Superior,  f2.  Inferior  frontal  fissures.  f3.  Sulcus  praecen- 
tralis.  PI.  Superior  parietal  lobule.  P2.  Inferior  parietal  lobule,  con- 
sisting of  P2.  Supramarginal  gyrus.  and  P/.  Angular  gyrus.  ip.  Sulcus 
interparietalis.  cm.  Termination  of  callosomarginal  fissure.  Oi.  First, 
O2.  Second,  O3.  Third  occipital  convolutions,  po.  Parieto-occipital  fissure. 
o.  Transverse  occipital  fissure.  o2.  Inferior  longitvidinal  occipital  fissure. 
T±.  First,  T2.  Second,  T3.  Temporosphenoid,  convolutions,  ti,  t2.  First,  t2. 
Second,  temporosphenoid  fissures. 


THE   CEREBRUM.  205 

tinuous  above  with  the  anterior  frontal,  and  below  with  the  inferior 
frontal,  convolution. 

The  superior  frontal  convolution  is  bounded  internally  by  the  longi- 
tudinal fissure,  and  externally  by  the  superior  frontal  fissure ;  it  is 
connected  with  the  superior  end  of  the  frontal  convolution,  and  runs 
downward  and  forward  to  the  anterior  extremity  of  the  frontal  lobe, 
where  it  turns  backward,  and  rests  upon  the  orbital  plate  of  the 
frontal  bone. 

The  middle  frontal  convolution,  the  largest  of  the  three,  runs 
from  behind  forward,  along  the  sides  of  the  lobe,  to  its  anterior 
part ;  it  is  bounded  above  by  the  superior  and  below  by  the  inferior 
frontal  fissures. 

The  inferior  frontal  convolution  winds  around  the  ascending 
branch  of  the  fissure  of  Sylvius,  in  the  anterior  and  inferior  portion 
of  the  cerebrum. 

Parietal  Lobe. — The  ascending  parietal  convolution  is  situated  just 
behind  the  fissure  of  Rolando,  running  downward  and  forward ; 
above,  it  becomes  continuous  with  the  upper  parietal  convolution,  and 
below,  winds  around  to  be  united  with  the  ascending  frontal. 

The  upper  parietal  convolution  is  situated  between  the  parietal 
and  longitudinal  fissures. 

The  supramarginal  convolution  winds  around  the  superior  extremity 
of  the  fissure  of  Sylvius. 

The  angular  convolution,  a  continuation  of  the  preceding,  follows 
the  parietal  fissure  to  its  posterior  extremity,  and  then  makes  a 
sharp  angle  downward  and  forward. 

Temporosphenoid  Lobe. — Contains  three  well-marked  convolutions, 
the  superior,  middle  and  inferior,  separated  by  well-defined  fissures, 
and  continuous  posteriorly  with  the  convolutions  of  the  parietal  lobe. 

The  occipital  lobe  lies  behind  the  parieto-occipital  fissure,  and 
contains  the  superior,  middle  and  inferior  convolutions,  not  well 
marked. 

The  central  lobe,  or  island  of  Reil,  situated  at  the  bifurcation  of 
the  fissure  of  Sylvius,  is  a  triangular-shaped  cluster  of  six  convolu- 
tions, the  gyri  operti,  which  are  connected  with  those  of 'the  frontal, 
parietal,  and  temporosphenoid  lobes. 

Upon  the  inner  or  mesial  aspect  of  the  hemisphere  are  found 
(Fig.  25)— 


206 


HUMAN    PHYSIOLOGY. 


1.  The  paracentral  lobule,  lying  in  the  region  of  the  upper  extremity 
of  the  fissure  of  Rolando  ;  it  contains  the  large  giant  cells  of  Betz. 
Injury    to    this    convolution    is    followed    by    degeneration    of    the 
motor  tract. 

2.  The    gyrus    fornicatus,    lying    below    the    callosomarginal    fissure. 
Running   parallel    with    the    corpus    callosum,    it   terminates    at    its 
posterior  border  in  the  hippocampal  gyrus. 


FIG.  25. — DIAGRAM  SHOWING  FISSURES  AND  CONVOLUTIONS  ON  MESIAL  ASPECT 
OF  THE  RIGHT  HEMISPHERE. 

Median  aspect  of  the  right  hemisphere.  CC.  Corpus  callosum  divided  longi- 
tudinally. Gf.  Gyrus  fornicatus.  H.  Gyrus  hippocampi,  h.  Sulcus  hippo- 
campi. U.  Uncinate  gyrus.  cm.  Callosomarginal  fissure.  FI.  First 
frontal  convolution,  c.  Terminal  portion  of  fissure  of  Rolando.  A.  As- 
cending frontal,  B.  Ascending  parietal,  convolution  and  paracentral  lobule. 
PI'.  Praecuneus  or  quadrate  lobule.  Oz.  Cuneus.  Po.  Parieto-occipital 
fissure,  o.  Transverse  occipital  fissure,  oc.  Calcarine  fissure.  ocf.  Su- 
perior, oc".  Inferior,  rami  of  the  same.  D.  Gyrus  descendens.  T*. 
Gyrus  occipitotemppralis  lateralis  (lobulus  fusiformis).  T5.  Gyrus  occipi- 
totemporalis  medialis  (lobulus  lingualis). 

3.  The  gyrus  hippocampus   (H)   is  formed  by  the  union  of  the  pre- 
ceding convolution  with  the  occipitotemporal.     It  runs  forward  and 
terminates  in  a  hooked  extremity — uncus. 

4.  The    quadrate    lobule,    or   pracuneus,   lies   between   the   upper   ex- 
tremity of  the  callosomarginal  fissure  and  the  parieto-occipital. 


THE   CEREBRUM.  207 

5.  The  cuneus  lies  posteriorly  to  the  quadrate  lobule.     It  is  a  wedge- 
shaped  mass  inclosed  by  the  calcarine  and  parieto-occipital  fissures. 

Structure. — The  gray  matter  of  the  cerebrum,  about  %  of  an 
inch  thick,  is  composed  of  five  layers  of  nerve-cells  : 

1.  A    superficial   layer,    containing   a    few    small    multipolar   ganglion 
cells. 

2.  Small  ganglion  cells,  pyramidal  in  shape. 

3.  A  layer  of  large  pyramidal  ganglion  cells  with  processes  running 
off  superiorly  and  laterally. 

4.  The  granular  formation  containing  nerve-cells. 

5.  Spindle-shaped  and  branching  nerve-cells  of  a  moderate  size. 
The  white  matter  consists  of  three  distinct  sets  of  fibers. 

1.  The   diverging  or  peduncular  fibers   are  mainly  derived   from  the 
columns    of    the    cord    and    medulla    oblongata ;    passing    upward 
through   the   crura   cerebri   they  receive   accessory   fibers   from   the 
olivary  fasciculus,  corpora  quadrigemina,  and  cerebellum.     Some  of 
the    fibers    terminate    in    the    optic    thalami    and    corpora    striata, 
while  others  radiate  into  the  anterior,  middle,  and  posterior  lobes 
of  the  cerebrum. 

2.  The   transverse   commissural  fibers   connect   the   two   hemispheres, 
through    the    corpus    callosum    and    anterior    and    posterior    com- 
missures. 

3.  The  longitudinal  commissural  fibers  connect  different  parts  of  the 
same  hemisphere. 

Functions. — The  cerebral  hemispheres  are  the  centers  of  the 
nervous  system  through  which  are  manifested  all  the  phenomena  of 
the  mind ;  they  are  the  centers  in  which  impressions  are  registered 
and  reproduced  subsequently  as  ideas ;  they  are  the  seat  of  intelli- 
gence, reason,  and  will. 

However  important  a  center  the  cerebrum  may  be  for  the  exhibi- 
tion of  this  highest  form  of  nervous  action,  it  is  not  directly  essential 
for  the  continuance  of  life,  for  it  does  not  exert  any  control  over 
those  automatic  reflex  acts,  such  as  respiration,  circulation,  etc., 
which  regulate  the  functions  of  organic  life. 

From   the   study    of   comparative    anatomy,    pathology,    vivisection, 
etc.,  evidence  has  been  obtained  which  throws  some  light  upon  the 
physiology  of  the  cerebral  hemispheres, 
i.  Comparative   anatomy   shows    that    there    is    a   general    connection 

between  the  size  of  the  brain,  its  texture,  the  depth  and  number 


208 


HUMAN    PHYSIOLOGY. 


FIG.  26. — SIDE  VIEW  OF  THE  BRAIN  OF  MAN,  WITH  THE  AREAS  OF  THE  CERE- 
BRAL CONVOLUTIONS  ACCORDING  TO  FERRIER. 

The  figures  are  constructed  by  marking  on  the  brain  of  man,  in  their 
respective  situations,  the  areas  of  the  brain  of  the  monkey  as  determined  by 
experiment,  and  the  description  of  the  effects  of  stimulating  the  various  areas 
refers 'to  the  brain  of  the  monkey. 

i.  Advance  of  the  opposite  hind  limb,  as  in  walking.  2,  3,  4.  Complex  move- 
ments of  the  opposite  leg  and  arm,  and  of  the  trunk,  as  in  swimming. 
a,  b,  c,  d.  Individual  and  combined  movements  of  the  fingers  and  wrists 
of  the  opposite  hand.  Prehensile  movements.  5.  Extension  forward  of 
the  opposite  arm  and  hand.  6.  Supination  and  flexion  of  the  opposite 
forearm.  7.  Retraction  and  elevation  of  the  opposite  angle  of  the  mouth, 
by  means  of  the  zygomatic  muscles.  8.  Elevation  of  the  alae  nasi  and 
upper  lip,  with  depression  of  the  lower  lip  on  the  opposite  side.  9,  10. 
Opening  of  the  mouth,  with  (9)  protrusion  and  (10)  retraction  of  the 
tongue;  region  of  aphasia,  bilateral  action.  u.  Retraction  of  the  op- 
posite angle  of  the  mouth,  the  head  turned  slightly  to  one  side.  12.  The 
eyes  open  widely,  the  pupils  dilate,  and  the  head  and  eyes  turn  toward 
the  opposite  side.  13,  13'.  The  eyes  move  toward  the  opposite  side  with 
an  upward  (13)  or  downward  (13')  deviation.  The  pupils  are  generally 
contracted.  14.  Pricking  of  the  opposite  ear,  the  head  and  eyes  turn  to 
the  opposite  side,  and  the  pupils  dilate  widely. 


CEREBRAL  LOCALIZATION   OF  FUNCTION.  209 

of  convolutions,  and  the  exhibition  of  mental  power.  Throughout 
the  entire  animal  series,  the  increase  in  intelligence  goes  hand  in 
hand  with  an  increase  in  the  development  of  the  brain.  In  man 
there  is  an  enormous  increase  in  size  over  that  of  the  highest 
animals,  the  anthropoids.  The  most  cultivated  races  of  men  have 
the  greatest  cranial  capacity ;  that  of  the  educated  European  being 
about  116  cubic  inches,  that  of  the  Australian  being  about  60  cubic 
inches,  a  difference  of  56  cubic  inches.  Men  distinguished  for 
great  mental  power  usually  have  large  and  well-developed  brains  ; 
that  of  Cuvier  weighed  64  ounces  ;  that  of  Abercrombie,  63  ounces ; 
the  average  being  about  48  to  50  ounces.  Not  only  the  size,  but, 
above  all,  the  texture,  of  the  brain  must  be  taken  into  consideration. 

2.  Pathology. — Any  severe  injury  or  disease  disorganizing  the  hemi- 
spheres   is    at   once   attended   by    a   disturbance    or   an    entire    sus- 
pension of  mental   activity.     A  blow  on  the  head,  producing  con- 
cussion,   or    undue    pressure    from    cerebral    hemorrhage,    destroys 
consciousness  ;  physical  and  chemic  alterations  in  the  gray  matter 

.  have  been  shown  to  coexist  with  insanity,  and  "with  loss  of  memory, 
speech,  etc.  Congenital  defects  of  organization  from  imperfect 
development  are  usually  accompanied  by  a  corresponding  deficiency 
of  intellectual  power  and  of  the  higher  instincts.  Under  these 
circumstances  no  great  advance  in  mental  development  can  be 
possible,  and  the  intelligence  remains  of  a  low  grade.  In  con- 
genital idiocy  not  only  is  the  brain  of  small  size,  but  it  is  wanting 
in  proper  chemic  composition,  phosphorus,  a  characteristic  in- 
gredient of  the  nervous  tissue,  being  largely  diminished  in  amount. 

3.  Experimentation  upon  the  lower  animals — e.  g.,  the  removal  of  the 
cerebral   hemispheres,   is   attended   by   results   similar  to   those   ob- 
served   in    disease    and    injury.       Removal    of    the    cerebrum    in 
pigeons  produces   complete   abolition   of   intelligence,   and   destroys 
the  capability  of  performing  spontaneous  movements.     The  pigeon 
remains  in  a  condition  of  profound  stupor,  which  is  not  accompanied, 
however,    by   a   loss    of   sensation    or    of   the   power    of   producing 
reflex  or  instinctive  movements.     The  -pigeon   can   be  temporarily 
aroused  by  pinching  the  feet,  loud  noises,  lights  placed  before  the 
eyes,  etc.,  but  soon  relapses  into  a  state  of  quietude,  being  unable 
to    remember    impressions    and    connect    them    with    any    train    of 
ideas,  the   faculties  of  memory,  reason,   and  judgment  being  com- 
pletely abolished. 

15 


210  HUMAN   PHYSIOLOGY. 


CEREBRAL  LOCALIZATION  OF  FUNCTIONS. 

From  experiments  made  upon  animals,  and  from  the  results  of 
clinical  and  post-mortem  observations  upon  men,  it  has  been  shown 
that  the  phenomena  of  organic  and  psychic  life  are  presided  over 
by  anatomically  localized  centers  in  the  brain.  A  knowledge  of  the 
position  of  these  centers  becomes  of  the  highest  importance  in  local- 
izing the  seat  of  lesions,  thrombi,  hemorrhages,  new  growths,  etc., 
which  show  themselves  in  paralyses,  epilepsies,  etc.  It  has  not 
been  possible  to  thus  localize  all  functions,  and  to  many  parts  of 
the  brain  no  special  use  can  be  assigned.  The  following  are  the  cen- 
ters most  definitely  mapped  out  and  that  are  of  paramount  im- 
portance. 

Motor  Centers. — These  are  in  the  cortical  gray  matter,  and  are 
arranged  along  either  side  of  the  fissure  of  Rolando.  This  area  is 
known  as  the  motor  area  or  motor  zone,  stimulation  of  which  is 
followed  by  convulsive  movements  of  the  muscles  of  the  opposite 
side  of  the  body,  while  destruction  of  the  gray  matter  of  this  area 
is  followed  by  permanent  paralysis  of  the  muscles  of  the  opposite 
side.  From  experiments  made  upon  monkeys,  Ferrier  has  mapped 
out  a  number  of  motor  centers  which  he  has  transformed  to  corres- 
ponding localities  on  the  human  brain.  (See  Fig.  26.)  The  de- 
scriptive text  of  the  illustration  renders  his  results  intelligible. 
Pathologic  studies  have  largely  confirmed  his  deductions.  In  a  gen- 
eral way  it  may  be  said  that  the  upper  third  of  the  ascending  frontal 
and  parietal  convolutions  about  this  fissure  preside  over  the  move- 
ments of  the  leg  of  the  opposite  side  of  the  body ;  the  middle  third 
controls  the  movements  of  the  arm ;  the  upper  part  of  the  inferior 
third  is  the  facial  area.  The  lowest  part  of  the  inferior  third  gov- 
erns the  motility  of  the  lips  and  tongue,  and  this  space,  with  the 
posterior  extremity  of  the  third  frontal  convolution,  constitutes 
the  speech  center. 

The  experiments  of  Horsley  and  Schafer  have  enabled  them  to 
furnish  a  new  diagrammatic  representation  of  the  motor  area  and 
more  accurately  to  define  the  special  areas  upon  the  lateral  and 
mesial  aspects  of  the  brain  of  the  monkey.  The  boundaries  of  the 
general  and  special  areas,  as  determined  by  these  observers,  will  be 
readily  understood  by  an  examination  of  Figures  27  and  28. 


CEREBRAL  LOCALIZATION   OF  FUNCTION. 


211 


For  diagnostic  purposes  the  motor  areas   for  the  face   and  limbs 
have  been  subdivided  as  follows : 
i.  The  face  area  may  be  divided  into  an  upper  part,  comprising  about 

one  third,  and  a  lower  part,  comprising  the  remaining  two  thirds. 


FIG.    27. — DIAGRAM    OF   THE    MOTOR    AREAS    ON    THE    OUTER    SURFACE    OF    A 
MONKEY'S  BRAIN. — (Horsley  and  Schafer.) 


FIG.  28. — DIAGRAM  OF  THE  MOTOR  AREAS  ON  THE  MARGINAL  CONVOLUTION  OF  A 
MONKEY'S  BRAIN. — (Horsley  and  Schafer.) 

In  the  upper  part  are  centers  governing  the  movements  of  the 
muscles  of  the  opposite  angle  of  the  mouth  and  of  the  lower  face. 
The  anterior  portion  of  the  lower  two  thirds  controls  the  move- 
ments of  the  vocal  cords,  and  may  be  regarded  as  a  laryngeal  cen- 


212  HUMAN   PHYSIOLOGY. 

ter ;  the  posterior  portion  governs  the  opening  and  shutting  of  the 
mouth  and  the  protrusion  and  retraction  of  the  tongue. 

2.  The  upper  limb   area  may   be   subdivided  as   follows :    The  upper 
part  controls  the  movements  of  the  shoulder ;  posteriorly  and  be- 
low  this   point   are   centers   for   the   elbow ;    below   and   anteriorly, 
centers  for  the  wrist  and  finger  movements,  while  lowest  and  pos- 
teriorly, centers  governing  the  thumb. 

3.  The   leg  area  may   be   subdivided   as   follows :    The   anterior  part, 
both  on  the  mesial  and  lateral  surfaces,  contains  centers  governing 
the   hip   and   thigh   movements ;    in   the   posterior  part   are   centers 
for  the  movements  of  the  leg  and  toes.     The  center  for  the  great 
toe  has  been  located  in  the  paracentral  lobule. 

4.  The   trunk   area,  situated  largely  on  the  mesial   surface,   contains 
anteriorly    centers    governing    the    rotation    and    arching    of    the 
spine,  while  posteriorly  are  found  centers  governing  movements  of 
the  tail  and  pelvis. 

5.  The  head  area,  or  area  for  visual  direction,  contains  centers  ex- 
citation of  which   causes   "  opening  of  the   eyes,   dilatation   of  the 
pupils,  and  turning  of  the  head  to  the  opposite  side,  with  conju- 
gate deviation  of  the  eyes  to  that  side." 

The  centers  of  origin  of  the  nerves  for  the  ocular  muscles  lie  in 
the  gray  matter  of  the  aqueduct  of  Sylvius.  Destruction  of  the 
gray  matter  at  these  points  is  followed  by  paralysis  of  the  muscles 
of  the  opposite  side  of  the  body,  and  morbid  growths,  hemorrhages, 
or  thrombi  of  the  vessels  of  the  part  result  in  abnormal  stimulation 
or  in  interference  with  the  functions  corresponding  to  the  nature 
and  extent  of  the  lesion.  Cerebral  or  Jacksonian  epilepsy  is  a 
result  of  local  cortical  disease. 

Center  for  Speech. — Pathologic  investigations  have  demonstrated 
that  the  left  third  frontal  convolution  is  of  essential  importance  for 
speech.  Adjoining  this  convolution  are  the  centers  controlling  the 
motility  of  the  lips,  tongue,  etc.  In  the  majority  of  cases  the  speech 
centers  are  on  the  left  side  of  the  brain,  though  in  exceptional 
cases  they  are  on  the  right  side,  especially  in  left-handed  persons. 
In  deaf  mutes  this  convolution  is  very  imperfectly  developed,  while 
in  monkeys  it  is  quite  rudimentary. 

Lesions  of  the  third  frontal  convolution  on  the  left  side,  if  the 
patient  be  right-handed,  produce  the  various  forms  of  aphasia,  or 
the  partial  or  complete  loss  of  the  power  of  articulate  speech. 


CEREBRAL   LOCALIZATION    OF   FUNCTION. 


213 


Aphasia  is  of  many  degrees  and  kinds.  In  ataxic  aphasia  the  pa- 
tient is  unable  to  communicate  his  thoughts  by  words,  there  being 
an  inability  to  execute  the  movements  of  the  mouth,  etc.,  necessary 
for  speech.  In  agraphic  aphasia  there  is  an  inability  to  execute  the 
movements  necessary  for  writing,  though  the  mental  processes 
are  retained.  In  the  ataxic  form  the  lesion  is  in  the  third  frontal 
convolution,  and  in  the 
agraphic  form  it  is  in  the 
arm  center. 

In  amnesic  aphasia  there 
is  a  loss  of  the  memory 
of  words,  the  purest  exam- 
ples of  which  consist  of 
the  affections  known  as 
word-deafness  and  word- 
blindness.  In  word-deaf- 
ness the  patient  can  not 
understand  vocal  speech, 
though  he  is  capable  of 
hearing  other  sounds. 
This  condition  is  associ- 
ated with  lesion  of  the 
first  temporal  convolution. 
In  word-blindness  the  pa- 
tient can  not  name  a 
letter  or  a  word  printed 
or  written,  though  he  can 
see  all  other  objects.  This 
condition  is  associated 
with  impairment  of  the 
visual  centers. 

Figure  29  will  illustrate 
the  conditions  in  the 
various  forms  of  aphasia. 

Impressions  are  constantly  passing  from  eye  and  ear  to  the  visual 
and  auditory  centers  and  are  there  being  registered.  Commissural 
fibers  connect  these  centers  with  the  arm  and  speech  centers,  which 
in  turn  are  connected  by  efferent  fibers  with  the  muscles  of  the  hand 
and  of  the  vocal  apparatus.  Muscular  movements  of  the  eye,  hand, 
and  mouth  are  also  registered  by  means  of  the  afferent  fibers,  s,  s',  s". 


FIG.  29. 


214  HUMAN    PHYSIOLOGY. 

Sensor  Centers. — These  are  the  centers  in  which  the  afferent  im- 
pulses are  translated  into  conscious  sensations.  The  most  im- 
portant are : 

The  visual  center,  located  in  the  occipital  lobe  and  especially  in 
the  cuneus.  Unilateral  destruction  of  this  area  results  in  hemianopsia, 
or  blindness  of  the  corresponding  halves  of  the  two  retinae.  Destruc- 
tion of  both  occipital  lobes  in  man  results  in  total  blindness.  Stimu- 
lation or  irritation  of  the  visual  center  causes  photopsia,  or  hallucina- 
tions of  sight,  in  corresponding  halves  of  the  retinae.  There  have 
been  instances  of  injury  of  these  parts  when  sensations  of  color 
were  abolished  with  preservation  of  those  of  space  and  light,  thus 
showing  a  special  localization  of  the  color  center.  Recent  experi- 
ments show  that  the  centers  of  the  two  hemispheres  are  united,  as 
ocular  fatigue  of  an  unused  eye  was  found  to  be  proportional  to  the 
fatigue  of  the  exercised  one. 

The  auditory  centers  are  located  in  the  temporosphenoid  lobes. 
Word-deafness  is  associated  with  softening  of  these  parts,  and  their 
complete  removal  results  in  deafness. 

The  gustatory  and  olfactory  centers  are  located  in  the  uncinate 
gyrus,  on  the  inner  side  of  the  temporosphenoid  lobes.  There  does 
not  seem  to  be  any  differentiation,  up  to  this  time,  of  these  two 
centers. 

The  center  for  tactile  impressions  was  located  by  Ferrier  in  the 
hippocampal  region.  Horsley  and  Schafer  found  that  destructive 
lesions  of  the  gyrus  fornicatus  were  followed  by  hemianesthesia  of 
the  opposite  side  of  the  body,  which  was  more  or  less  marked  and 
persistent.  These  observers  conclude  that  the  limbic  lobe  "  is 
largely,  if  not  exclusively,  concerned  in  the  appreciation  of  sensa- 
tions, painful  and  tactile." 

The  superior  and  middle  frontal  convolutions  appear  to  be  the 
seats  of  the  reason,  intelligence,  and  will.  Destruction  of  these  parts 
is  followed  by  proportional  hebetude,  without  any  impairment  of 
sensation  or  motion. 


THE   SYMPATHETIC   NERVE   SYSTEM.  215 

THE    SYMPATHETIC    NERVE    SYSTEM. 

The  sympathetic  nerve  system  consists  of  a  chain  of  ganglia 
connected  by  longitudinal  nerve  filaments,  situated  on  each  side  of 
the  spinal  column,  running  from  above  downward.  The  two  gangli- 
onic  cords  are  connected  in  the  interior  of  the  cranium  by  the  ganglion 
of  Ribes,  on  the  anterior  communicating  artery,  and  terminate  in  the 
ganglion  impar,  situated  at  the  top  of  the  coccyx. 

The  chain  of  ganglia  is  divided  into  groups,  and  named  according 
to  the  location  in  which  they  are  found — viz.,  cranial,  four  in  num- 
ber ;  cervical,  three ;  thoracic,  twelve ;  lumbar,  five ;  sacral,  five ; 
coccygeal,  one.  Each  ganglion  consists  of  a  collection  of  vesicular 
nervous  matter,  bundles  of  non-medullated  nerve-fibers,  embedded  in 
a  capsule  of  connective  tissue.  The  ganglia  are  reinforced  by  motor 
and  sensory  fibers  from  the  cerebro-spinal  nervous  system. 

The  ganglia  have  distinct  nerve-fibers,  from  which  branches  are 
distributed  to  the  glands,  arteries,  and  muscles,  and  to  the  cerebral 
and  spinal  nerves,  many  pass,  also  the  visceral  ganglia — e.  g.,  cardiac, 
semilunar,  pelvic,  etc. 

Cephalic  Ganglia. 

1.  The    ophthalmic    or    ciliary    ganglion    is    situated    in    the    orbital 
cavity,  posterior  to  the  eyeball ;  it  is  of  small  size  and  of  a  reddish- 
gray   color ;   receives  filaments   of  communication   from   the  motqjpl 
oculi  ophthalmic  branch  of  the  fifth  pair,  and  the  carotid  plexus. 
Its    filaments    of    distribution    are    the    ciliary    nerves,    which    con- 
sist of — 

(a)  Motor  fibers  for  the  circular  fibers   of  the  iris   and  ciliary 
muscle. 

(b)  Sensor  fibers  for  the  cornea,  iris,  and  associated  parts. 

(c)  Vaso-motor  fibers  for  the  blood-vessels  of  the  choroid,  iris, 
and  retina. 

(d}  Motor  fibers  for  the  dilator  fibers  of  the  iris. 

2.  The  spheno-palatine  or  Meckel's  ganglion,  triangular  in  shape,  is 
situated  in  the  sphenomaxillary  fossa ;  receives  filaments  from  the 
facial    (Vidian   nerve)    and   the   superior   maxillary   branch   of   the 
fifth   nerve.      Its   filaments   of   distribution   pass   to    the   gums,    the 
soft  palate  and  associated  parts. 

3.  The  otic  or  Arnold's  ganglion  is  of  small  size,  oval  in  shape,  and 
situated  beneath  the  foramen  ovale  ;  receives  a  motor  filament  from 


216  HUMAN  PHYSIOLOGY. 

the  facial  and  sensor  filaments  from  the  glossopharyngeal  and  fifth 
nerves ;  sends  filaments  to  the  mucous  membrane  of  the  tympanic 
cavity  and  to  the  tensor  tympani  muscle. 

4.  The  submaxillary  ganglion,  situated  in  the  submaxillary  gland, 
receives  filaments  from  the  chorda  tympani,  sensor  filaments  from 
the  lingual  branch  of  the  fifth  nerve,  and  filaments  from  the  sym- 
pathetic. The  chorda  tympani  nerve  supplies  vaso-dilator  and 
secretor  fibers  to  the  submaxillary  and  sublingual  glands.  The 
fifth  nerve  endows  the  glands  with  sensibility,  while  the  sympa- 
thetic supplies  vaso-constrictor  fibers  to  the  blood-vessels  of  the 
glands. 

Cervical  Ganglia. 

The  superior  cervical  ganglion  is  fusiform  in  shape,  of  a  grayish- 
red  color,  and  situated  opposite  the  second  and  third  cervical  ver- 
tebra ;  it  sends  branches  to  form  the  carotid  and  cavernous  plexuses, 
which  branches  follow  the  course  of  the  carotid  arteries  to  their 
distribution;  also  sends  branches  to  join  the  glossopharyngeal  and 
pneumogastric,  to  form  the  pharyngeal  plexus. 

The  middle  cervical  ganglion,  the  smallest  of  the  three,  is  oc- 
casionally absent ;  it  is  situated  opposite  the  fifth  cervical  vertebra ; 
sends  branches  to  the  superior  and  inferior  cervical  ganglia  and  to  the 
thyroid  artery. 

The  inferior  cervical  ganglion,  irregular  in  form,  is  situated  oppo- 
site the  last  cervical  vertebra ;  it  is  frequently  united  with  the  first 
thoracic  ganglion. 

The  superior,  middle,  and  inferior  cardiac  nerves,  arising  from 
these  cervical  ganglia,  pass  downward  and  forward  to  form  the  deep 
and  superficial  cardiac  plexuses  located  at  the  bifurcation  of  the 
trachea,  from  which  branches  are  distributed  to  the  heart,  coronary 
arteries,  etc. 

The  thoracic  ganglia  are  usually  twelve  in  number,  and  are 
placed  against  the  heads  of  the  ribs  behind  the  pleura ;  they  are 
small  in  size  and  gray  in  color ;  they  communicate  with  the  cerebro- 
spinal  nerves  by  two  filaments,  one  of  which  is  white,  the  other  gray. 

The  great  splanchnic  nerve  is  formed  by  the  union  of  branches 
from  the  sixth,  seventh,  eighth,  and  ninth  ganglia  ;  it  passes  through 
the  diaphragm  to  the  semilunar  ganglion. 

The  lesser  splanchnic  nerve  is   formed  by  the  union  of  filaments 


THE    SYMPATHETIC    NERVE    SYSTEM.  217 

from  the  tenth  and  eleventh  ganglia,  and  is  distributed  to  the  celiac 
plexus. 

The  renal  splanchnic  nerve  arises  from  the  last  thoracic  ganglion 
and  terminates  in  the  renal  plexus. 

The  semilunar  ganglia,  the  largest  of  the  sympathetic  system,  are 
situated  by  the  side  of  the  celiac  axis ;  they  send  radiating  branches 
to  form  the  solar  plexus;  from  the  various  plexuses,  nerves  follow 
the  gastric,  splenic,  hepatic,  renal,  etc.,  arteries,  into  the  different 
abdominal  viscera. 

The  lumbar  ganglia,  four  in  number,  are  placed  upon  the  bodies 
of  the  vertebra ;  they  give  off  branches,  which  unite  to  form  the 
aortic  himbar  plexus  and  the  hypogastric  plexus,  and  follow  the 
blood-vessels  to  their  terminations. 

The  sacral  and  coccygeal  ganglia  send  filaments  of  distribution 
to  all  the  blood-vessels  of  the  pelvic  viscera. 

Properties  and  Functions. — The  sympathetic  nerve  possesses  both 
sensibility  and  the  power  of  exciting  motion,  but  these  properties 
are  much  less  decided  than  in  the  cerebro-spinal  system.  Division 
of  the  sympathetic  nerve  in  the  neck  is  followed  by  a  vascular  con- 
gestion of  the  parts  above  the  section  on  the  corresponding  side, 
attended  by  an  increase  in  the  temperature ;  not  only  is  there  an 
increase  in  the  amount  of  blood,  but  the  rapidity  of  the  blood  current 
is  very  much  accelerated  and  the  blood  in  the  veins  becomes  of  a 
brighter  color.  Galvanization  of  the  upper  end  of  the  divided  nerve 
causes  all  the  preceding  phenomena  to  disappear;  the  congestion 
decreases,  the  temperature  falls,  and  the  venous  blood  becomes 
dark  again. 

The  sympathetic  exerts  a  similar  influence  upon  the  circulation 
of  the  limbs  and  the  glandular  organs  ;  destruction  of  the  first  thoracic 
ganglion  and  division  of  the  nerves  forming  the  lumbar  and  sacral 
plexuses  are  followed  by  a  dilatation  of  the  vessels,  an  increased 
rapidity  of  the  circulation,  and  an  elevation  of  temperature  in  the 
anterior  and  posterior  limbs ;  galvanization  of  the  peripheral  ends 
of  these  nerves  causes  all  of  these  phenomena  to  disappear.  Division 
of  the  splanchnic  nerve  causes  a  dilatation  of  the  blood-vessels  of 
the  intestine. 

These  phenomena  of  the  sympathetic  nervous  system  are  dependent 
upon  the  presence  of  vaso-motor  nerves,  which,  under  normal  cir- 
cumstances, exert  a  tonic  influence  upon  the  blood-vessels.  These 


218  HUMAN   PHYSIOLOGY. 

nerves,  derived  from  the  cerebro-spinal  system  and  the  medulla 
oblongata,  leave  the  spinal  cord  by  the  rami  communicant es,  enter 
the  sympathetic  ganglia,  and  finally  terminate  in  the  muscular  walls 
of  the  blood-vessels. 


THE    CRANIAL    NERVES. 

The  cranial  nerves  come  off  from  the  base  of  the  brain,  pass 
through  foramina  in  the  walls  of  the  cranium,  and  are  distributed 
to  the  structures  of  the  head,  the  face  and  in  part  to  the  organs  of 
the  thorax  and  abdomen. 

According  to  the  classification  of  Soemmering,  there  are  twelve 
pairs  of  nerves,  enumerating  them  from  before  backward,  as  fol- 
lows— viz. : 

First  pair,  or  olfactory.  Seventh  pair  or  facial,  portio  dura. 

Second  pair  or  optic.  Eighth   pair,   or   auditory,   portio 

Third  pair,  or  motor  oculi  com-           mollis. 

munis.  Ninth  pair,  or  glosso-pharyngeal. 
Fourth  pair,  or  patheticus,  troch-       Tenth  pair,  or  pneumogastric. 

learis.  Eleventh  pair,  or  spinal  accessory. 

Fifth  pair,  or  trigeminal.  Twelfth  pair,  or  hypoglossal. 
Sixth  pair,  or  abducens. 

The  cranial  nerves  may  also  be  classified  physiologically,  according 
to  their  function,  into  three  groups  : 

1.  Nerves  of  special  sense — e.  g.,  olfactory,  optic,  auditory,  gustatory 
(glosso-pharyngeal  and  chorda  tympani). 

2.  Nerves   of   motion — e.   g.,   motor   oculi,   patheticus,    small    root   of 
the  trigeminal,  facial,  spinal  accessory  and  hypoglossal. 

3.  Nerves  of  general  sensibility — e.  g.,  large  root  of  the  trigeminal,  the 
glosso-pharyngeal  and  the  pneumogastric. 

ORIGINS    OF   THE   CRANIAL    NERVES. 

The  nerves  of  special  sense  have  their  origin  in  neuro-epithelial 
cells  in  the  sense  organs  with  which  they  are  connected. 

The  nerves  of  motion  have  their  origin  in  nerve  cells  situated  in 
the  gray  matter  beneath  the  floor  of  the  aqueduct  of  Sylvius  and 
the  floor  of  the  fourth  ventricle. 


THE  CRANIAL   NERVES.  219 

The  nerves  of  general  sensibility  have  their  origin  in  the  ganglia 
situated  on  their  trunks. 

First  Pair.     Olfactory. 

The  olfactory  nerve  is  situated  in  the  upper  third  of  the  nasal 
fossa.  It  consists  of  from  20  to  30  branches. 

Origin. — From  neuro-epithelial  cells  situated  among  the  epithelial 
cells  covering  the  mucous  membrane.  From  these  cells  the  nerve- 
fibers  pass  upward  through  foramina  in  the  cribriform  plate  of  the 
ethmoid  bone  and  arborize  around  nerve-cells,  in  the  olfactory  bulb. 

The  Olfactory  Tract. — The  olfactory  tract  consists  of  both  gray 
and  white  fibers  which  pass  from  their  origin  in  the  bulb,  to  the 
base  of  the  cerebrum  where  it  divides  into  three  branches,  viz.,  an 
external  white  root,  which  passes  across  the  fissure  of  Sylvius  to  the 
middle  lobe  of  the  cerebrum ;  an  internal  white  root,  which  passes 
also  into  the  middle  lobe  ;  a  gray  root,  which  is  in  relation  with  the 
anterior  lobe.  The  white  fibers  at  least  terminate  around  nerve-cells 
in  the  gray  matter  of  the  pre-callosal  part  of  the  gyrus  fornicatus, 
the  gyrus  hippocampus  and  the  gyrus  uncinatus. 

Properties. — The  olfactory  nerves  do  not  give  rise  to  either  motor 
or  sensor  phenomena  when  stimulated.  When  stimulated  at  their 
periphery  by  odorous  particles,  nerve  impulses  are  developed  which, 
when  conducted  to  the  brain,  evoke  the  sensation  of  smell.  Destruction 
of  the  olfactory  nerves,  the  bulb  or  tract,  is  followed  by  a  loss  of  the 
sense  of  smell. 

Function. — Presides  over  the  sense  of  smell.  Conducts  impulses 
to  the  cerebrum  which  give  rise  to  odorous  sensations. 

Second  Pair.    Optic. 

Origin. — The  optic  nerve  arises  from  large  nerve-cells  in  the 
anterior  part  of  the  retina.  From  this  origin  the  nerve-fibers  turn 
backward  and  converge  to  form  a  well-defined  bundle  (the  optic 
nerve)  which  passes  out  of  the  eyeball,  through  the  orbit  cavity 
as  far  as  the  sella  turcica.  At  this  point  there  is  a  union  and  partial 
decussation,  in  man  at  least,  of  the  fibers,  forming  what  is  known 
as  the  optic  chiasm.  From  the  posterior  portion  of  the  chiasm  there 
passes  backward  on  either  side  a  bundle  of  nerve-fibers,  the  optic 
tract.  Each  tract  contains  nerve-fibers  which  come  from  the  temporal 


220  HUMAN   PHYSIOLOGY. 

two  thirds  of  the  retina  of  the  same  side  and  the  nasal  third  of  the 
retina  of  the  opposite  side.  The  fibers  of  the  optic  tract  arborize 
around  nerve-cells  in  the  external  geniculate  body,  the  pulvinar,  and 
the  anterior  quadrigeminal  body.  By  means  of  the  optic  radiation, 
the  nerve-cells  in  these  different  ganglia  are  brought  into  relation 
with  the  visual  center,  the  cuneus. 

Properties. — The  optic  nerves  are  insensible  to  ordinary  impres- 
sions, and  convey  only  the  special  impressions  of  light.  Division 
of  one  of  the  nerves  is  attended  by  complete  blindness  in  the  eye  of 
the  corresponding  side. 

Hemiopia  and  Hemianopsia. — Owing  to  the  decussation  of  the 
fibers  in  the  optic  chiasm,  division  of  the  optic  tract  produces  loss  of 
sight  in  the  outer  half  of  the  eye  of  the  same  side,  and  in  the  inner 
half  of  the  eye  of  the  opposite  side,  the  blind  part  being  separated 
from  the  normal  part  by  a  vertical  line.  The  term  hemiopia  is 
applied  to  the  loss  of  function  or  paralysis  of  the  one  half  of  the 
retina ;  as  a  result  of  this,  there  will  be  an  obliteration  of  the  field 
of  vision  on  the  opposite  side  to  which  the  term  hemianopsia  is 
given.  If,  for  example,  che  right  optic  tract  be  divided,  there  will 
be  hemiopia  in  the  outer  half  of  the  right  eye  and  inner  half  of  the 
left  eye,  thus  causing  left  lateral  hemianopsia,  and  as  the  two 
halves  are  affected  which  correspond  in  normal  vision,  the  condition 
is  known  as  homonymous  hemianopsia.  Lesion  of  the  anterior  part 
of  the  optic  chiasm  causes  blindness  in  the  inner  half  of  the  two 
eyes. 

Functions. — Governs  the  sense  of  sight.  Receives  and  conveys  to 
the  brain  the  nerve  impulses  made  by  ether  vibrations  and  which 
give  rise  to  the  sensation  of  light. 

The  reflex  movements  of  the  iris  are  called  forth  by  stimulation 
of  the  optic  nerve.  When  light  falls  upon  the  retina,  the  nerve  im- 
pulse developed  is  carried  back  to  the  tubercula  quadrigemina,  where 
it  is  transformed  into  a  motor  impulse,  which  then  passes  outward 
through  the  motor  oculi  nerve  to  the  contractile  fibers  of  the  iris  and 
diminishes  the  size  of  the  pupil.  The  absence  of  light  is  followed  by 
a  dilatation  of  the  pupil. 

Third  Pair.     Motor  Oculi  Communis. 

Origin. — From  several  groups  of  nerve-cells  situated  in  the  gray 
matter  beneath  the  aqueduct  of  Sylvius. 


THE   CRANIAL   NERVES.  221 

Distribution. — From  this  origin  the  nerve-fibers  pass  forward  and 
emerge  from  the  cerebrum  at  the  inner  side  of  the  cms  cerebri.  The 
nerve  then  passes  forward,  and  enters  the  orbit  through  the  sphenoid 
fissure,  where  it  divides  into  a  superior  branch  distributed  to  the 
superior  rectus  and  levator  palpebra  muscles ;  an  inferior  branch, 
sending  branches  to  the  internal  and  inferior  recti  and  the  inferior 
oblique  muscles ;  filaments  also  pass  into  the  ciliary  or  ophthalmic 
ganglion ;  from  this  ganglion  the  ciliary  nerves  arise,  which  enter 
the  eyeball  and  are  distributed  to  the  circular  fibers  of  the  iris  and  the 
ciliary  muscle.  The  third  nerve  also  receives  filaments  from  the 
cavernous  plexus  of  the  sympathetic  and  from  the  fifth  nerve. 

Properties. — Irritation  of  the  root  of  the  nerve  produces  contraction 
of  the  pupil,  internal  strabismus,  and  muscular  movements  of  the 
eye,  but  no  pain.  Division  of  the  nerve  is  followed  by  ptosis  (falling 
of  the  upper  eyelid)  ;  external  strabismus,  due  to  the  unopposed  ac- 
tion of  the  external  rectus  muscle  ;  paralysis  of  the  accommodation 
of  the  eye ;  dilatation  of  the  pupil  from  paralysis  of  the  circular 
fibers  of  the  iris  and  ciliary  muscle ;  and  inability  to  rotate  the  eye, 
slight  protrusion,  and  double  vision.  The  images  are  crossed ;  that 
of  the  paralyzed  eye  is  a  little  above  that  of  the  second,  and  its  upper 
end  inclined  toward  it. 

Function. — Governs  movements  of  the  eyeball  by  innervating  all 
the  muscles  except  the  external  rectus  and  superior  oblique,  influ- 
ences the  movements  of  the  iris,  elevates  the  upper  lid,  influences  the 
accommodation  of  the  eye  for  distances.  Can  be  called  into  action 
by  (i)  voluntary  stimuli,  (2)  by  reflex  action  through  irritation  of  the 
optic  nerve. 

Fourth  Pair.    Patheticus. 

Origin. — From  nerve-cells  situated  in  the  gray  matter  beneath  the 
aqueduct  of  Sylvius,  just  posterior  to  the  last  nucleus  of  the  third 
nerve. 

Distribution. — The    nerve    enters    the    orbital    cavity    through    the 
sphenoid  fissure,  and  is   distributed  to  the  superior  oblique  muscle  ; 
.  in  its  course  it  receives  filaments  from  the  ophthalmic  branch  of  the 
fifth  pair  and  the  sympathetic. 

Properties. — When  the  nerve  is  irritated,  muscular  movements 
are  produced  in  the  superior  oblique  muscle,  and  the  pupil  of  the 
eye  is  turned  dozvnward  and  outward.  Division  or  paralysis  lessens 


222  HUMAN   PHYSIOLOGY. 

the  movements  and  rotation  of  the  globe  downward  and  outward. 
The  diplopia  consequent  upon  this  paralysis  is  homonymous,  one 
image  appearing  above  the  other.  The  image  of  the  paralyzed  eye 
is  below,  its  upper  end  inclined  toward  that  of  the  sound  eye. 

Function. — Governs  the  movements  of  the  eyeball  produced  by  the 
action  of  the  superior  oblique  muscles. 

Sixth  Pair.*    Abducens.     Motor  Oculi  Externus. 
Origin. — From  nerve-cells   situated  beneath  the  upper  half  of  the 
floor  of  the  fourth  ventricle. 

Distribution. — From  this  origin  the  nerve  passes  into  the  orbit 
through  the  sphenoid  fissure,  and  is  distributed  to  the  external  rectus 
muscle.  Receives  filaments  from  the  cervical  portion  of  the  sympa- 
thetic, through  the  carotid  plexus,  and  spheno-palatine  ganglion. 

Properties. — When  irritated,  the  external  rectus  muscle  is  thrown 
into  convulsive  movements  and  the  eyeball  is  turned  outward.  When 
divided  or  paralyzed,  this  muscle  is  paralyzed,  motion  of  the  eyeball 
outward  past  the  median  line  is  impossible,  and  the  homonymous 
diplopia  increases  as  the  object  is  moved  outward  past  this  line. 
The  images  are  upon  the  same  plane  and  parallel.  Internal  strabismus 
results  because  of  the  unopposed  -action  of  the  internal  rectus. 

Function. — To  innervate  the  external  rectus  muscle  by  which  the 
eyeball  is  turned  outward. 

Fifth  Pair.     Trigeminal. 

The  fifth  nerve  consists  of  both  afferent  and  efferent  fibers  which 
for  the  most  part  are  separate  and  distinct.  The  afferent  fibers  con- 
stitute by  far  the  major  portion,  the  efferent  fibers  the  minor  portion  of 
the  nerve. 

Origin  of  the  Afferent  Fibers. — The  afferent  fibers  have  their 
origin  in  nerve-cells  in  the  Gasserian  ganglion.  From  each  cell  a 
short  process  develops  which  soon  divides  into  two  branches,  one  of 
which  passes  centrally,  the  other  peripherally.  The  centrally  di- 
rected branches  form  the  so-called  large  root;  the  peripherally  di-  • 
rected  branches  collectively  constitute  the  three  main  divisions  of  the 

*  The  sixth  nerve  is  considered  in  connection  with  the  third  and  fourth 
nerves  since  they  together  constitute  the  motor  apparatus  by  which  the 
ocular  muscles  are  excited  to  action. 


THE   CRANIAL    NERVES.  223 

nerve,  viz. :  the  ophthalmic,  the  superior  maxillary  and  the  inferior 
maxillary. 

Distribution. — The  centrally  directed  branches  enter  the  pons 
Varolii  on  its  lateral  aspect.  After  pursuing  a  short  distance,  these 
fibers  arborize  around  nerve-cells  in  the  gray  matter  of  the  pons 
and  medulla. 

The  peripherally  directed  branches  are  distributed  as  follows : 

1.  The  ophthalmic  branches  to  the  conjunctiva  and  skin  of  the  upper 
eyelid,    the   cornea,    the   skin    of   the    forehead   and   the    nose,    the 
lachrymal  gland  and  the  mucous  membrane  of  the  nose. 

2.  The    Superior    maxillary    branches    to    the    skin    and    conjunctiva 
of   the   lower   lid,   the   nose,   the   cheek   and   upper   lip,    the   palate 
teeth  of  the  upper  jaw  and  the  alveolar  processes. 

3.  The  inferior  maxillary  branches  to  the  external  auditory  meatus, 
the  side  of  the  head,  the  mouth,  the  tongue,  the  teeth  of  the  lower 
jaw,  the  alveolar  processes  and  the  skin  of  the  lower  part  of  the 
face. 

Properties. — The  trigeminal  nerve,  composed  mainly  of  afferent 
fibers,  is  the  most  acutely  sensitive  nerve  in  the  body,  and  endows  all 
the  parts  to  which  it  is  distributed  with  general  sensibility. 

Irritation  of  the  large  root,  or  of  any  of  its  branches,  will  give  rise 
to  marked  evidence  of  pain  ;  the  various  forms  of  neuralgia  of  the 
head  and  face  being  occasioned  by  compression,  disease,  or  exposure 
of  some  of  its  terminal  branches. 

Division  of  the  large  root  within  the  cranium  is  followed  at  once 
by  a  complete  abolition  of  all  sensibility  in  the  head  and  face,  but  is 
not  attended  by  any  loss  of  motion.  The  integument,  the  mucous 
membranes,  and  the  eye  may  be  lacerated,  cut,  or  bruised,  without 
the  animal  exhibiting  any  evidence  of  pain.  At  the  same  time  the 
lachrymal  secretion  is  diminished,  the  pupil  becomes  contracted,  the 
eyeball  is  protruded,  and  the  sensibility  of  the  tongue  is  abolished. 

The  reflex  movements  of  deglutition  are  also  somewhat  impaired, 
the  impression  of  the  food  being  unable  to  reach  and  excite  the  nerve 
center  in  the  medulla  oblongata. 

Origin  of  the  Efferent  Fibers. — The  efferent  fibers  have  their 
origin  in  nerve-cells  in  the  gray  matter  of  the  pons  Varolii  beneath 
the  floor  of  the  fourth  ventricle. 

Distribution. — The  efferent  fibers,  known  collectively  as  the  small 
root,  emerge  from  the  side  of  the  pons  Varolii,  pass  forward  beneath 


224  HUMAN   PHYSIOLOGY. 

the  ganglion  of  Gasser,  beyond  which  they  enter  the  inferior  max- 
illary division.  After  a  short  course  most  of  these  fibers  leave  the 
common  trunk  and  are  distributed  to  the  muscles  of  mastication, 
viz. :  the  temporal,  the  masseter,  the  internal  and  external  pterygoid 
muscles.  Other  fibers  are  distributed  to  the  mylohyoid  muscle,  the 
tensor  palati  and  the  tensor  tympani  muscles. 

Properties. — Stimulation  of  the  small  root  produces  convulsive 
movements  of  the  muscles  of  mastication ;  section  of  the  root  causes 
paralysis  of  these  muscles,  after  which  the  jaw  is  drawn  to  the 
opposite  side  by  the  action  of  the  opposing  muscles. 

The  Influence  of  the  Trigeminal  on  the  Special  Senses. — After 
division  of  the  large  root  within  the  cranium,  a  disturbance  in  the 
nutrition  of  the  special  senses  sooner  or  later  manifests  itself. 

Sight. — In  the  course  of  twenty-four  hours  the  eye  becomes  very 
vascular  and  inflamed,  the  cornea  becomes  opaque  and  ulcerates,  the 
humors  are  discharged,  and  the  eye  is  totally  destroyed. 

Smell. — The  nasal  mucous  membrane  swells  up,  becomes  fungous, 
and  is  liable  to  bleed  on  the  slightest  irritation.  The  mucus  is 
increased  in  amount,  so  as  to  obstruct  the  nasal  passages ;  the  sense 
of  smell  is  finally  abolished. 

Hearing. — At  times  the  hearing  is  impaired  from  disorders  of  nu- 
trition in  the  middle  ear  and  external  auditory  meatus. 

Alteration  in  the  nutrition  of  the  special  senses  is  not  marked 
if  the  section  is  made  posterior  to  the  ganglion  of  Gasser  and  to  the 
anastomosing  filaments  of  the  sympathetic,  which  join  the'  nerves  at 
this  point ;  but  if  the  ganglion  be  divided,  these  effects  are  very 
noticeable,  owing  to  the  section  of  the  sympathetic  filaments. 

Function. — The  trigeminal  nerve,  through  its  afferent  fibers,  en- 
dows all  the  parts  of  the  head  and  face  to  which  it  is  distributed 
with  sensibility ;  through  its  efferent  fibers  it  gives  motion  to  the 
muscles  of  mastication,  and  to  the  tensor  muscle  of  the  palate  and 
the  tensor  of  the  tympanic  membrane  ;  through  anastomosing  fibers 
from  the  sympathetic  it  influences  the  nutrition  of  the  special  senses. 

Seventh  Pair.     Portio  Dura.     Facial  Nerve. 

Origin. — From  a  large  nucleus  of  nerve-cells  situated  in  the  gray 
matter  beneath  the  upper  half  of  the  floor  of  the  fourth  ventricle. 

Distribution. — From  this  origin  the  nerve  emerges  from  the  lower 
border  of  the  pons.  It  then  passes  into  the  internal  auditory  meatus 


THE   CRANIAL   NERVES.  225 

in  company  with  the  nerve  of  Wrisberg,  and  then  enters  the  aqueduct 
of  Fallopius. 

The  nerve-fibers  composing  the  nerve  of  Wrisberg  have  their 
origin  in  nerve-cells  in  the  geniculata  ganglion,  situated  on  the  facial 
just  where  it  bends  to  enter  the  aqueduct  of  Fallopius.  The  cen- 
trally directed  branches  enter  the  medulla  oblongata  around  the 
nerve-cells,  in  which  they  terminate ;  the  peripherally  directed 
branches  enter  the  trunk  of  the  facial. 

In  the  aqueduct  the  facial  gives  off  the  following  branches — viz. : 

1.  The   large   petrosal   nerve,   which   passes    forward   to    the   spleno- 
palatine,  or  Meckel's  ganglion. 

2.  The  small  petrosal  nerve,  which  passes  to  the  otic  ganglion. 

3.  The   tympanic  branch,  which  passes  to   the   stapedius   muscle   and 
endows  it  with  motion. 

4.  The    chorda    tympani   nerve,    which,    after    entering   the    posterior 
part  of  the  tympanic  cavity,   passes   forward  between  the  malleus 
and   incus,   through   the   Glasserian   fissure,    and   joins   the   lingual 
branch   of  the  fifth  nerve.      It   is   then   distributed   to   the  mucous 
membrane  of  the  anterior  two  thirds  of  the  tongue  and  the  sub- 
maxillary  glands. 

After  emerging  from  the  stylomastoid  foramen,  the  facial  nerve 
sends  branches  to  the  muscles  of  the  ear,  the  occipitofrontalis,  the 
digastric,  the  palatoglossi,  and  palatopharyngei ;  after  which  it  passes 
through  the  parotid  gland  and  divides  into  the  temporofacial  and 
cervicofacial  branches,  which  are  distributed  to  the  superficial  muscles 
of  the  face — viz.,  occipitofrontalis,  corrugator  supercilii,  orbicularis 
palpebrarum,  levator  labii  superioris  et  alaeque  nasi,  buccinator,  levator 
anguli  oris,  orbicularis  oris,  zygomatici,  depressor  anguli  oris,  platysma 
myoides,  etc. 

Properties. — Undoubtedly  a  motor  nerve  at  its  origin,  but  in 
its  course  receives  sensitive  filaments  from  the  fifth  pair  and  the 
pneumogastric. 

Irritation  of  the  nerve,  after  its  emergence  from  the  stylomastoid 
foramen,  produces  convulsive  movements  in  all  the  superficial  muscles 
of  the  face.  Division  of  the  nerve  at  this  point  causes  paralysis  of 
these  muscles  on  the  side  of  the  section,  constituting  facial  paralysis, 
the  phenomena  of  which  are  a  relaxed  and  immobile  condition  of 
the  same  side  of  the  face  ;  the  eyelids  remain  open,  from  paralysis 
of  the  orbicularis  palpebrarum;  the  act  of  winking  is  abolished;  the 
16 


226  HUMAN    PHYSIOLOGY. 

angle  of  the  mouth  droops,  and  saliva  constantly  drains  away;  the 
face  is  drawn  over  to  the  second  side ;  the  face  becomes  distorted 
upon  talking  or  laughing ;  mastication  is  interfered  with,  the  food 
accumulating  between  the  gums  and  cheek,  from  paralysis  of  the 
buccinator  muscle  ;  fluids  escape  from  the  mouth  in  drinking ;  articula- 
tion is  impaired,  the  labial  sounds  being  imperfectly  pronounced. 

Properties  and  Functions  of  the  Branches  Given  off  in  the 
Aqueduct  of  Fallopius. 

1.  The  large  petrosal,  when  stimulated,  gives  rise  to  a  dilatation  of 
the  blood-vessels   and   a  secretion   from  the  mucous  membrane   of 
nose,   soft  palate,  upper  part  of  the  pharynx,   roof   of  the  mouth, 
and  gums.      It   therefore   contains   vaso-motor   and  secretor  fibers, 
which  are  in  relation  with  the  spheno-palatine  ganglion. 

2.  The  tympanic  branch  causes  the  stapedius  muscle  to  contract. 

3.  The  chorda  tympani  influences  the  circulation  of  the  blood  around 
and    the    secretion    of    saliva    from    the    submaxillary    glands,    and 
through  the  nerve  of  Wrisberg  governs  the  sense  of.  taste  in  the 
anterior   two   thirds   of   the   tongue.      Galvanization   of   the   chorda 
tympani  dilates  the  blood-vessels,  increases  the  quantity  and  rapidity 
of   the    stream    of    blood,    and    increases    the    secretion    of    saliva. 
Division   of  the   nerve   is   followed  by   contraction   of  the   vessels, 
an  arrest  of  the  secretion,  and  a  diminution  of  the  sense  of  taste 
on  the  same  side.     It  therefore  contains  vaso-motor,  secretor  and 
gustatory  nerve-fibers. 

Function. — The  facial  is  the  nerve  of  expression,  and  coordinates 
the  muscles  employed  to  delineate  the  various  emotions,  influences  the 
sense  of  taste  and  the  secretions  of  the  submaxillary  and  sublingual 
glands. 

Eighth  Pair.    Portio  Mollis.    Auditory  Nerve. 

The  eighth  nerve  consists  of  two  portions,  a  cochlear  or  auditory 
and  a  vestibular  or  equilibratory. 

Origin. — The  cochlear  portion  has  its  origin  in  the  bipolar  nerve- 
cells  of  the  spiral  ganglion  located  in  the  spiral  canal  near  the  base 
of  the  osseous  lamina  spiralis.  The  vestibular  portion  has  its  origin 
in  the  bipolar  nerve-cells  of  the  ganglion  of  Scarpa,  located  in  the 
internal  auditory  meatus. 

Distribution. — The  common  trunk  of  the  eighth  nerve,  consisting 
of  both  the  cochlear  and  vestibular  portions,  emerge  from  the 


THE   CRANIAL   NERVES.  227 

internal  auditory  meatus  after  which  it  passes  backward  and  inward 
as  far  as  the  lateral  aspect  of  the  pons,  where  the  two  main  divisions 
again  separate.  The  cochlear  portion  passes  to  the  outer  side  of  the 
restiform  body ;  the  vestibular  portion  passes  to  the  inner  side  of  the 
restiform  body  to  the  dorsal  portion  of  the  pons.  After  entering  the 
pons  the  fibers  composing  both  portions  come  into  histologic  rela- 
tions with  different  groups  of  nerve-cells. 

Properties. — Stimulation  of  the  cochlear  nerve  is  unattended  by 
either  motor  or  sensor  phenomena.  Division  of  the  nerve  is  followed 
by  a  loss  of  hearing.  Destruction  of  the  semicircular  canal,  involving 
a  lesion  of  the  vestibular  nerves  at  their  origin,  is  followed  by  a 
loss  of  the  power  of  coordination  and  equilibration. 

Functions. — The  cochlear  nerve  presides  over  the  sense  of  hearing. 
It  carries  to  the  brain  the  nerve  impulses  produced  by  the  impact  of 
atmospheric  vibrations  on  the  ear,  and  which  give  rise  to  the  sen- 
sation of  sound.  The  vestibular  nerve  carries  nerve  impulses  to 
the  brain,  which  excite  certain  reflex  adaptive  movements  by  which 
the  equilibrium  of  the  body  is  maintained. 

Ninth  Pair.     Glossopharyngeal. 

Origin. — From  nerve-cells  in  the  ganglia  situated  on  the  trunk 
of  the  nerve  near  the  medulla  oblongata — viz.,  the  petrosal  ganglion 
and  the  jugular  ganglion.  From  these  cells  a  single  branch  emerges, 
which  soon  divides  into  two  branches,  one  of  which  passes  centrally, 
the  other  peripherally.  The  centrally  directed  branches  enter  the 
medulla  oblongata,  where  they  terminate  around  nerve-cells.  The 
peripherally  directed  branches  collectively  form  the  two  main  di- 
visions from  which  the  nerve  takes  its  name. 

The  glossopharyngeal  also  contains  efferent  nerve-fibers,  which 
have  their  origin  in  nerve-cells  beneath  the  floor  of  the  fourth 
.ventricle. 

Distribution. — The  trunk  of  the  nerve  passes  downward  and  for- 
ward, receiving  near  the  jugular  ganglion  fibers  from  the  facial  and 
pneumogastric  nerves.  It  divides  into  two  large  branches,  one  of 
which  is  distributed  to  the  base  of  the  tongue,  the  other  to  the 
pharynx.  In  its  course  it  sends  filaments  to  the  otic  ganglion ;  a 
tympanic  branch  which  gives  sensibility  to  the  mucous  membrane  of 
the  fenestra  rotunda,  fenestra  ovalis,  and  Eustachian  tube ;  lingual 


228  HUMAN    PHYSIOLOGY. 

branches  to  the  base  of  the  tongue ;  palatal  branches  to  the  soft 
palate,  uvula,  and  tonsils ;  pharyngeal  branches  to  the  mucous  mem- 
brane of  the  pharynx. 

Properties. — Irritation  of  the  roots  at  their  origin  calls  forth 
evidences  of  pain ;  it  is,  therefore,  a  sensor  nerve,  but  its  sensibility 
is  not  so  acute  as  that  of  the  trigeminal.  Irritation  of  the  trunk  after 
its  exit  from  the  cranium  produces  contraction  of  the  muscles  of  the 
palate  and  pharynx,  owing  to  the  presence  of  motor  fibers. 

Division  of  the  nerve  abolishes  sensibility  in  the  structures  to 
which  it  is  distributed  and  impairs  the  sense  of  taste  in  the  posterior 
third  of  the  tongue  (see  Sense  of  Taste). 

Function. — Governs  the  sensibility  of  the  pharynx,  presides  partly 
over  the  sense  of  taste,  and  controls  reflex  movements  of  deglutition 
and  vomiting. 

Tenth  Pair.    Pneumogastric.    Vagus. 

Origin. — From  nerve-cells  situated  along  the  trunk  of  the  nerve 
near  the  medulla  oblongata — viz. :  the  jugular  and  the  plexiform 
ganglia.  From  the  nerve-cells  in  these  ganglia  a  short  process 
emerges  which  soon  divides  into  two  branches  one  of  which  passes 
centrally,  the  other  peripherally.  The  central  branches  enter  the 
medulla  oblongata,  where  they  terminate  around  nerve-cells ;  the 
peripheral  branches  collectively  form  the  main  portion  of  the  trunk 
of  the  nerve. 

The  pneumogastric  also  contains  efferent  fibers  which  have  their 
origin  in  nerve-cells  beneath  the  floor  of  the  medulla  oblongata.  It 
also  receives  motor  fibers  from  the  spinal  accessory,  the  facial,  the 
hypoglossal  and  the  anterior  branches  of  the  two  upper  cervical 
nerves. 

Distribution. — As  the  nerve  passes  down  the  neck  it  sends  off  the 
following  main  branches  : 

1.  Pharyngeal  nerves,  which  assist  in  forming  the  pharyngeal  plexus, 
which  is  distributed  to  the  mucous  membrane  and  to  the  muscles  of 
the  pharynx. 

2.  Superior   laryngeal   nerve,   which    enters    the    larynx    through    the 
thyrohyoid  membrane,  and  is  distributed  to  the  mucous  membrane 
lining  the  interior  of  the  larynx,  and  to  the  cricothyroid  muscle  and 
the  inferior  constrictor  of  the  pharynx.     The    '  depressor  nerve," 


THE   CRANIAL   NERVES.  229 

found  in  the  rabbit,  is  formed  by  the  union  of  two  branches,  one 
from  the  superior  laryngeal,  the  other  from  the  main  trunk ;  it 
passes  downward  to  be  distributed  to  the  heart. 

3.  Inferior   laryngeal,   which   sends   its   ultimate   branches   to   all   the 
intrinsic    muscles    of    the    larynx    except    the    cricothyroid,    and   to 
the  inferior  constrictor  of  the  pharynx. 

4.  Cardiac  branches  given  off  from  the  nerve  throughout  its  course, 
which  unite  with  the  sympathetic  fibers  to  form  the  cardiac  plexus, 
to  be  distributed  to  the  heart. 

5.  Pulmonary  branches,  which  form  a  plexus  of  nerves,  and  are  dis- 
tributed to  the  bronchi  and  their  ultimate  terminations,  the  lobules 
and  air-cells. 

From  the  right  pneumogastric  nerve  branches  are  distributed  to 
the  mucous  membrane  and  muscular  coats  of  the  stomach  and  intes- 
tines, and  to  the  liver,  spleen,  kidneys,  and  suprarenal  capsules. 

Properties. — At  its  origin  the  pneumogastric  nerve  is  sensory,  as 
shown  by  direct  irritation  or  galvanization,  though  its  sensibility  is 
not  very  marked.  In  its  course  it  exhibits  motor  properties,  from 
anastomosis  with  motor  nerves. 

The  pharyngeal  branches  assist  in  giving  sensibility  to  the  mucous 
membrane  of  the  pharynx,  and  influence  reflex  phenomena  of  deglu- 
tition through  motor  fibers  which  they  contain,  derived  from  the 
spinal  accessory. 

The  superior  laryngeal  nerve  endows  the  upper  portion  of  the 
larynx  with  sensibility;  protects  it  from  the  entrance  of  foreign 
bodies;  by  conducting  impressions  to  the  medulla,  excites  the  reflex 
movements  of  deglutition  and  respiration ;  through  the  motor  fila- 
ments it  contains,  produces  contraction  of  the  cricothyroid  muscle. 

Division  of  the  "  depressor  nerve "  and  galvanization  of  the 
central  end  retard  and  even  arrest  the  pulsations  of  the  heart,  and 
by  depressing  the  vaso-motor  center,  diminish  the  pressure  of  blood 
in  the  large  vessels,  by  causing  dilatation  of  the  intestinal  vessels 
through  the  splanchnic  nerves. 

The  inferior  laryngeal  contains,  for  the  most  part,  motor  fibers 
from  the  spinal  accessory.  When  irritated,  produces  movement  in 
the  laryngeal  muscles.  When  divided,  is  followed  by  paralysis  of 
these  muscles,  except  the  cricothyroid,  impairment  of  phonation,  and 
an  embarrassment  of  the  respiratory  movements  of  the  larynx,  and, 
finally,  death  from  suffocation. 


230  HUMAN   PHYSIOLOGY. 

The  cardiac  branches,  through  filaments  derived  from  the  spinal 
accessory,  or  possibly  from  the  medulla  oblongata  direct,  exert  a 
direct  inhibitory  action  upon  the  heart.  Division  of  the  pneumo- 
gastrics  in  the  neck  is  followed  by  increased  frequency  of  the 
heart's  action.  Galvanization  of  the  peripheral  ends  diminishes  the 
heart's  pulsations,  and,  if  sufficiently  powerful,  arrests  it  in  diastole. 

The  pulmonary  branches  give  sensibility  to  the  bronchial  mucous 
membrane  and  govern  the  movements  of  respiration.  Division  of 
both  pneumogastrics  in  the  neck  diminishes  the  frequency  of  the 
respiratory  movements,  which  may  fall  as  low  as  four  to  six  a  minute  ; 
death  usually  occurs  in  from  five  to  eight  days.  Feeble  galvanization 
of  the  central  ends  of  the  divided  nerves  accelerates  respiration ; 
powerful  galvanization  retards,  and  may  even  arrest,  the  respiratory 
movements. 

The  gastric  branches  give  sensibility  to  the  mucous  coat,  and 
through  motor  or  efferent  fibers  give  motion  to  the  muscular  coat  of 
the  stomach.  They  influence  the  secretion  of  gastric  juice,  and  aid 
the  process  of  digestion. 

The  hepatic  branches,  probably  through  anastomosing  sympathetic 
filaments,  influence  the  secretion  of  bile  and  the  glycogenic  function 
of  the  liver ;  division  of  the  pneumogastrics  in  the  neck  produces 
congestion  of  the  liver,  diminishes  the  density  of  the  bile,  and  arrests 
the  glycogenic  function ;  galvanisation  of  the  central  ends  exaggerates 
the  glycogenic  function  and  makes  the  animal  diabetic. 

The  intestinal  branches  give  sensibility  and  motion  to  the  small 
intestines. 

Function. — A  great  sensor  nerve,  which,  through  filaments  from 
motor  sources,  influences  deglutition,  the  action  of  the  heart,  the 
circulatory  and  respiratory  systems,  voice,  the  secretions  of  the 
stomach,  intestines,  and  various  glandular  organs,  and  the  contrac- 
tion of  the  walls  of  the  stomach  and  intestines. 

Eleventh  Pair.    Spinal  Accessory. 

The  spinal  accessory  nerve  consists  of  two  distinct  portions,  the 
medullary  or  bulbar,  and  the  spinal. 

Origin. — The  medullary  portion  has  its  origin  in  nerve-cells  in 
the  lower  part  of  the  nucleus  ambiguous,  located  beneath  the  floor  of 
the  fourth  ventricle.  From  this  origin  the  nerve-fibers  pass  forward 
and  emerge  from  the  medulla  oblongata  on  its  lateral  aspect. 


THE   CRANIAL   NERVES.  231 

The  spinal  portion  has  its  origin  in  nerve-cells  located  in  the 
lateral  gray  matter  of  the  spinal  cord  as  far  down  as  the  fifth  cer- 
vical nerve.  From  this  origin  the  nerve-fibers  pass  to  the  surface  of 
the  cord  to  emerge  between  the  ventral  and  dorsal  roots  in  from 
six  to  eight  filaments,  after  which  they  unite  to  form  a  well-defined 
nerve.  It  then  passes  into  the  cranial  cavity  through  the  foramen 
magnum  and  unites  with  the  medullary  portion. 

Distribution. — After  the  union  the  common  trunk  emerges  from 
the  cranial  cavity  through  the  jugular  foramen  and  after  sending 
branches  to  'the  pneumogastric  and  receiving  others  in  turn  from  the 
pneumogastric  as  well  as  from  the  upper  cervical  nerves  it  divides 
into  two  branches — viz. : 

1.  An  internal   or   anastomotic  branch   which   soon   enters   the  trunk 
of  the  pneumogastric   nerve.     The  fibers   of  this   branch   are  ulti- 
mately  distributed   to   some   of  the  pharyngeal   muscles ;   to   all   of 
the  muscles  of  the  larynx  by  way  of  the  laryngeal  branches  of  the 
vagus  nerve,   and,   according  to  most  authorities,  to  the  heart. 

2.  An    external    branch    consisting    chiefly    of    the    accessory    fibers 
from  the  spinal  cord.     It  is  distributed  to  the  sterno-cleido-mastoid 
and  trapezius  muscles. 

Properties. — At  its  origin  it  is  a  purely  motor  nerve,  but  in  its 
course  it  exhibits  some  sensibility,  due  to  the  presence  of  anasto- 
mosing fibers. 

Destruction  of  the  medullary  root — e.  g.,  tearing  it  from  its  at- 
tachment by  means  of  forceps,  impairs  the  action  of  the  muscles  of 
deglutition  and  destroys  the  power  of  producing  vocal  sounds  from 
paralysis  of  the  laryngeal  muscles,  without,  however,  interfering 
with  the  respiratory  movements  of  the  larynx,  these  being  controlled 
by  other  motor  nerves.  The  normal  rate  of  movement  of  the 
heart  is  increased  by  destruction  of  the  medullary  root. 

Irritation  of  the  external  branch  throws  the  trapezius  and  sterno- 
mastoid  muscles  into  convulsive  movements,  though  section  of  the 
nerve  does  not  produce  complete  paralysis,  as  they  are  also  supplied 
with  motor  influence  from  the  cervical  nerves.  The  sternomastoid 
and  trapezius  muscles  perform  movements  antagonistic  to  those  of 
respiration,  fixing  the  head,  neck,  and  upper  part  of  the  thorax,  and 
delaying  the  expiratory  movement  during  the  acts  of  pushing,  pulling, 
straining,  etc.,  and  in  the  production  of  a  prolonged  vocal  sound,  as 
in  singing.  When  the  external  branch  alone  is  divided,  in  animals, 


232  HUMAN   PHYSIOLOGY. 

they  experience  shortness  in  breath  during  exercise,  from  a  want  of 
coordination  of  the  muscles  of  the  limbs  and  respiration ;  and  while 
they  can  make  a  vocal  sound,  it  can  not  be  prolonged. 

Function. — Governs  phonation  by  its  influence  upon  the  muscles 
regulating  the  position  and  tension  of  the  vocal  bands  ;  influences  the 
movements  of  deglutition,  inhibits  the  action  of  the  heart,  and  con- 
trols certain  respiratory  movements  associated  with  sustained  or  pro- 
longed muscular  efforts  and  phonation. 

Twelfth  Pair.     Hypoglossal   or   Sublingual. 

Origin. — From  nerve-cells  situated  deep  in  the  substance  of  the 
medulla  oblongata,  on  a  level  with  the  lowest  portion  of  the  floor  of 
the  fourth  ventricle.  From  this  origin  the  fibers  pass  forward  and 
emerge  from  the  medulla  in  the  groove  between  the  anterior  pyramid 
and  the  olivary  body. 

Distribution. — The  trunk  formed  by  the  union  of  the  different 
filaments  passes  out  of  the  cranial  cavity  through  the  anterior  con- 
dyloid  foramen.  After  emerging  from  the  cranium,  it  sends  fila- 
ments to  the  sympathetic  and  pneumogastric ;  it  anastomoses  with 
the  lingual,  branch  of  the  fifth  pair,  and  receives  and  sends  filaments 
to  the  upper  cervical  nerves.  The  nerve  is  finally  distributed  to  the 
sternohyoid,  sternothyroid,  omohyoid,  thyrohyoid,  styloglossi,  hyo- 
glossi,  geniohyoid,  geniohyoglossi,  and  the  intrinsic  muscles  .of  the 
tongue. 

Properties. — A  purely  motor  nerve  at  its  origin,  but  derives  sensi- 
bility outside  the  cranial  cavity  from  anastomosis  with  the  cervical 
pneumogastric,  and  fifth  nerves. 

Irritation  of  the  nerve  gives  rise  to  convulsive  movements  of  the 
tongue  and  slight  evidences  of  sensibility. 

Division  of  the  nerve  abolishes  all  movements  of  the  tongue  and 
interferes  considerably  with  the  act  of  deglutition. 

When  the  hypoglossal  nerve  is  involved  in  hemiplegia,  the  tip  of 
the  tongue  is  directed  to  the  paralyzed  side  when  the  tongue  is  pro- 
truded, owing  to  the  unopposed  action  of  the  geniohyoglossus  on 
the  sound  side. 

Articulation  is  considerably  impaired  in  paralysis  of  this  nerve, 
great  difficulty  being  experienced  in  the  pronunciation  of  the  con- 
sonantal sounds. 


THE   SENSE  OF  TOUCH.  233 

Mastication  is  performed  with   difficulty,   from   inability   to   retain 
the  food  between  the  teeth  until  it  is  completely  triturated. 

Function. — Governs  all  the  movements  of  the  tongue  and  influences 
the  functions  of  mastication,  deglutition  and  articulation. 


THE    SENSE    OF    TOUCH. 

The  sense  of  touch  is  a  modification  of  general  sensibility,  and  is 
located  in  the  skin,  which  is  especially  adapted  for  this  purpose  on 
account  of  the  number  of  nerves  and  papillary  elevations  it  possesses. 
The  structures  of  the  skin  and  the  modes  of  termination  of  the 
sensory  nerves  have  already  been  considered. 

The  tactile  sensibility  varies  in  acuteness  in  different  portions  of 
the  body,  being  most  marked  in  those  regions  in  which  the  tactile 
corpuscles  are  most  abundant — e.  g.,  the  palmar  surface  of  the  third 
phalanges  of  the  fingers  and  thumb. 

The  relative  sensibility  of  different  portions  of  the  body  has  been 
ascertained  by  means  of  a  pair  of  compasses :  the  points  of  the 
instrument  being  guarded  by  cork,  it  was  then  determined  how 
closely  they  could  be  brought  together,  and  yet  be  felt  at  two  different 
points.  The  following  are  some  of  the  measurements : 

Point  of  tongue l/2  of  a  line. 

Palmar  surface  of  third  phalanx i  line. 

Red  surface  of  lips 2  lines. 

Palmar  surface  of  metacarpus 3 

Tip  of  the  nose 3 

Part  of  lips  covered  by  skin 4 

Palm  of  hand 5 

Lower  part  of  forehead 10 

Back  of  hand 14      " 

Dorsum  of  foot 18      " 

Middle  of  the  thigh 30      " 

The  sense  of  touch  communicates  to  the  mind  the  idea  of  resistance 
only,  and  the  varying  degrees  of  resistance  offered  to  the  sensory 
nerves  enable  us  to  estimate,  with  the  aid  of  the  muscular  sense,  the 
qualities  of  hardness  or  softness  of  external  objects.  The  idea  of 


234  HUMAN   PHYSIOLOGY. 

space  or  extension  is  obtained  when  the  sensory  surface  or  the  ex- 
ternal object  changes  its  place  in  regard  to  the  other;  the  character 
of  the  surface,  its  roughness  or  smoothness,  is  estimated  by  the  im- 
pressions made  upon  the  tactile  papillae. 

Appreciation  of  Temperature. — The  general  surface  of  the  body  is 
more  or  less  sensitive  to  differences  of  temperature,  though  this 
sensation  is  separate  from  that  of  touch ;  whether  there  are  nerves 
especially  adapted  for  the  conduction  of  this  sensation  has  not  been 
fully  determined.  Under  pathologic  conditions,  however,  the  sense 
of  touch  may  be  abolished,  while  the  appreciation  of  changes  in  tem- 
perature may  remain  normal. 

The  cutaneous  surface  varies  in  its  sensibility  to  temperature  in 
different  parts  of  the  body,  and  depends,  to  some  extent,  upon  the 
thickness  of  the  skin,  exposure,  habit,  etc. ;  the  inner  surface  of  the 
elbow  is  more  sensitive  to  changes  in  temperature  than  the  outer 
portion  of  the  arm ;  the  left  hand  is  more  sensitive  than  the  right, 
the  mucous  membrane  less  so  than  the  skin. 

Excessive  heat  or  cold  has  the  same  effect  upon  the  sensibility ; 
the  temperatures  most  readily  appreciated  are  those  between  50°  F. 
and  115°  F. 

The  sensations  of  pain  and  tickling  appear  to  be  conducted  to  the 
brain,  also,  by  nerves  different  from  those  of  touch ;  in  abnormal 
conditions  the  appreciation  of  pain  may  be  entirely  lost  while  touch 
remains  unimpaired. 


THE    SENSE    OF    TASTE. 

The  sense  of  taste  is  localized  mainly  in  the  mucous  membrane 
covering  the  superior  surface  of  the  tongue. 

The  tongue  is  situated  in  the  floor  of  the  mouth ;  its  base  is 
directed  backward  and  is  connected  with  the  hyoid  bone  and  by 
numerous  muscles  with  the  epiglottis  and  soft  palate;  its  apex  is 
directed  forward  against  the  posterior  surface  of  the  teeth. 

The  substance  of  the  tongue  is  made  up  of  intrinsic  muscle-fibers, 
the  linguales ;  it  is  attached  to  surrounding  parts,  and  its  various 
movements  are  performed  by  the  extrinsic  muscles — e.  g.,  stylo- 
glossus,  geniohyoglossus,  etc. 

The  mucous  membrane  covering  the  tongue  is  continuous  with  that 


THE    SENSE   OF   TASTE.  235 

lining  the  commencement  of  the  alimentary  canal,  and  is   furnished 
with  vascular  and  nervous  papillae. 

The  papilla?  are  analogous  in  their  structure  to  those  of  the  skin, 
and  are  distributed  over  the  dorsum  of  the  tongue,  giving  it  its  char- 
acteristic roughness. 

There  are  three  principal  varieties — 

1.  The  filiform  papilla  are  most  numerous,  and  cover  the  anterior  two 
thirds   of  the   tongue ;   they   are   conic   or  filiform   in   shape,   often 
prolonged  into   filamentous  tufts,   of  a  whitish   color,   and   covered 
by  horny  epithelium. 

2.  The  fungi  form  papilla  are  found  chiefly  at  the  tip   and  sides   of 
the  tongue ;  they  are  larger  than  the  preceding,  and  may  be  recog- 
nized by  their  deep  red  color. 

3.  The   circumvallate  papilla   are   rounded   eminences    from   eight   to 
ten    in   number,    situated    at   the    base    of   the    tongue,    where   they 
form   a   V-shaped  figure.      They  are  quite  large,   and   consist  of  a 
central    projection    of    mucous    membrane,    surrounded    by    a    wall, 
or  circumvallation,  from  which  they  derive  their  name. 

The  taste-beakers,  supposed  to  be  the  true  organs  of  taste,  are 
flask-like  bodies,  ovoid  in  form,  about  ¥^0-  of  an  inch  in  length, 
situated  in  the  epithelial  covering  of  the  mucous  membrane,  on  the 
circumvallate  papillae.  They  consist  of  a  number  of  fusiform,  narrow 
cells,  which  are  curved  so  as  to  form  the  walls  of  this  flask-like  body  ; 
in  the  interior  are  elongated  cells,  with  large,  clear  nuclei,  the 
taste-cells. 

Nerves  of  Taste. — The  chorda  tympani  nerve,  a  branch  of  the 
facial,  after  leaving  the  cavity  of  the  tympanum,  joins  the  third 
division  of  the  fifth  nerve  between  the  two  pterygoid  muscles,  and 
then  passes  forward  in  the  lingual  branches,  to  be  distributed  to  the 
mucous  membrane  of  the  anterior  two  thirds  of  the  tongue.  Division 
or  disease  of  this  nerve  is  followed  by  a  loss  of  taste  in  the  part  to 
which  it  is  distributed. 

The  glossopharyngeal  enters  the  tongue  at  the  posterior  border  of 
the  hyoglossus  muscle,  and  is  distributed  to  the  mucous  membrane  of 
the  base  and  sides  of  the  tongue,  fauces,  etc. 

The  lingual  branch  of  the  trifacial  nerve  endows  the  tongue  with 
general  sensibility  ;  the  hypoglossal  endows  it  with  motion. 


236  HUMAN   PHYSIOLOGY. 

The  nerves  of  taste  in  the  superficial  layer  of  the  mucous  membrane 
form  a  fine  plexus,  from  which  branches  pass  to  the  epithelium  and 
penetrate  it ;  others  enter  the  taste-beakers,  and  are  directly  con- 
nected with  the  taste-cells. 

The  seat  of  the  sense  of  taste  has  been  shown  by  experiment  to 
be  the  whole  of  the  mucous  membrane  over  the  dorsum  of  the  tongue, 
soft  palate,  fauces,  and  upper  part  of  the  pharynx. 

The  sense  of  taste  enables  us  to  distinguish  the  savor  of  sub- 
stances introduced  into  the  mouth,  which  faculty  is  different  from 
tactile  sensibility.  The  sapid  qualities  of  substances  appreciated  by 
the  tongue  are  designated  as  bitter,  sweet,  alkaline,  sour,  salt,  etc. 

The  essential  conditions  for  the  production  of  the  impressions  of 
taste  are : 

1.  A  state  of  solubility  of  the  food. 

2.  A  free  secretion  of  the  saliva,  and 

3.  Active   movements   on   the  part   of  the   tongue,    exciting   pressure 
against  the  roof  of  the  mouth,  gums,  etc.,  thus  aiding  the  solution 
of  various  articles  and  their  osmosis  into  the  lingual  papillae. 
Sapid  substances,  when  in  a  state  of  solution,  pass  into  the  interior 

of  the  taste-beakers,  and  come  into  contact,  through  the  medium  of 
the  taste-cells,  with  the  terminal  filaments  of  the  gustatory  nerves. 


THE    SENSE    OF    SMELL. 

The  sense  of  smell  is  located  in  the  mucous  membrane  lining  the 
upper  part  of  the  nasal  cavity,  in  which  the  olfactory  nerves  are 
distributed. 

The  nasal  fossae  are  two  cavities,  irregular  in  shape,  separated  by 
the  vomer,  the  perpendicular  plate  of  the  ethmoid  bone,  and  the  tri- 
angular cartilage.  They  open  anteriorly  and  posteriorly  by  the  an- 
terior and  posterior  nares,  the  latter  communicating  with  the  pharynx. 
They  are  lined  by  mucous  membrane,  of  which  the  only  portion 
capable  of  receiving  odorous  impressions  is  the  part  lining  the  upper 
one  third  of  the  fossae. 

The  olfactory  nerves,  the  olfactory  bulb  and  tracts,  unite  the  ol- 
factory epithelium  with  the  cortical  areas  of  smell  in  the  cerebrum. 

In  animals  which  possess  an  acute  sense  of  smell  there  is  a  cor- 
responding increase  in  the  development  of  the  olfactory  bulbs. 


THE   SENSE   OF    SIGHT.  237 

The  essential  conditions  for  the  sense  of  smell  are — 
i.  A  special  nerve  center  capable  of  receiving  impressions  and  trans- 
forming them   into   odorous   sensations. 
2..  Emanations    from    bodies    which    are    in    a    gaseous    or    vaporous 

condition. 

3.  The  odorous  emanations  must  be  drawn  freely  through  the  nasal 
fossae ;  if  the  odor  be  very  faint,  a  peculiar  inspiratory  movement  is 
made,  by  which  the  air  is  forcibly  brought  into  contact  with  the 
olfactory  filaments.  The  secretions  of  the  nasal  fossae  probably  dis- 
solve the  odorous  particles. 

Various  substances,  as  ammonia,  horseradish,  etc.,  excite  the  sensi- 
bility of  the  mucous  membrane ;  this  must  be  distinguished  from  the 
perception  of  true  odors. 


THE    SENSE    OF    SIGHT. 

The  Eyeball. — The  eyeball,  or  organ  of  vision,  is  situated  at  the 
fore  part  of  the  orbital  cavity  and  is  supported  by  a  cushion  of  fat ; 
it  is  protected  from  injury  by  the  bony  walls  of  the  cavity,  the  lids, 
and  the  lashes,  and  is  so  situated  as  to  permit  of  an  extensive  range 
of  vision.  The  eyeball  is  loosely  held  in  position  by  a  fibrous  mem- 
brane, the  capsule  of  Tenon,  which  is  attached  on  the  one  hand 
to  the  eyeball  itself  and  on  the  other  to  the  walls  of  the  cavity. 
Thus  suspended,  the  eyeball  is  capable  of  being  moved  in  any  direc- 
tion by  the  contraction  of  the  muscles  attached  to  it. 

Structure. — The  eyeball  is  spheroid  in  shape  and  measures  about 
T90-  of  an  inch  in  its  anteroposterior  diameter,  and  a  little  less  in  its 
transverse  diameter.  When  viewed  in  profile,  it  is  seen  to  consist 
of  the  segments  of  two  spheres,  of  which  the  posterior  is  the  larger, 
occupying  five  sixths,  and  the  anterior  the  smaller,  occupying  one 
sixth,  of  the  ball. 

The  eye  is  made  up  of  several  membranes,  concentrically  arranged, 
within  which  are  inclosed  the  refracting  media  essential  to  vision. 
These  membranes,  enumerated  from  without  inward,  are — 

1.  The  sclerotic  and  cornea. 

2.  The  choroid  and  iris. 

3.  The  retina. 

The  refracting  media  are  the  aqueous  humor,  the  crystalline  lens, 
and  the  vitreous  humor. 


238  HUMAN    PHYSIOLOGY. 

The  Sclerotic  and  Cornea.— The  sclerotic  is  the  opaque  fibrous 
membrane  covering  the  posterior  five  sixths  of  the  ball.  It  is  com- 
posed of  connective  tissue  arranged  in  layers,  which  run  both  trans- 
versely and  longitudinally ;  it  is  pierced  posteriorly  by  the  optic  nerve 
about  TVof  an  inch  internal  to  the  optic  axis.  The  sclerotic,  by  its 
density,  gives  form  to  the  eye  and  protects  the  delicate  structures 
within  it,  and  serves  for  the  attachment  of  the  muscles  by  which  the 
ball  is  moved. 

The  cornea  is  a  transparent  non-vascular  membrane  covering  the 
anterior  one  sixth  of  the  eyeball.  It  is  nearly  circular  in  shape  and  is 
continuous  at  the  circumference  with  the  sclerotic,  from  which  it 
can  not  be  separated.  The  substance  of  the  cornea  is  made  up  of 
thin  layers  of  delicate,  transparent  fibrils  of  connective  tissue,  more 
or  less  united ;  between  these  layers  are  found  a  number  of  inter- 
communicating lymph-spaces,  lined  by  endothelium,  which  are  in 
connection  with  lymphatics.  Leukocytes  or  lymph-corpuscles  are 
often  found  in  these  spaces.  The  anterior  surface  of  the  cornea 
is  covered  by  several  layers  of  nucleated  epithelium,  which  rest  upon 
a  structureless  membrane  known  as  the  anterior  elastic  lamina.  The 
posterior  surface  is  covered  by  a  similar  membrane,  the  membrane 
of  Descemet,  which  at  its  periphery  becomes  continuous  with  the  iris  ; 
it  is  also  covered  by  a  layer  of  epithelial  cells.  At  the  junction  of 
the  cornea  and  sclerotic  is  found  a  circular  groove,  the  canal  of 
Schlemm. 

The  choroid,  the  iris,  the  ciliary  muscle,  and  the  ciliary  processes 
together  constitute  the  second  or  middle  coat  of  the  eyeball. 

The  choroid  is  a  dark  brown  membrane  which  extends  forward 
nearly  to  the  cornea,  where  it  terminates  in  a  series  of  folds,  the 
ciliary  processes.  In  its  structure  the  choroid  is  highly  vascular, 
consisting  of  both  arteries  and  veins.  Externally  it  is  connected  with 
the  sclerotic  by  connective  tissue  ;  internally  it  is  lined  by  a  layer  of 
hexagonal  pigment  cells,  which,  though  usually  classed  as  belonging 
to  the  choroid,  is  now  known  to  belong,  embryologically  and  physio- 
logically, to  the  retina.  From  without  inward  may  be  distinguished 
the  following  layers  : 

1.  The  lamina  suprachoroidea. 

2.  The  elastic  layer  of  Sattler,  consisting  of  two  endothelial  layers. 

3.  The  chorio-capillaris,  choroid  proper,  or  membrane  of  Ruysch — a 
thick,    elastic    network    of    arterioles    and    capillaries    lying    within 
the  outer  layer  of  veins  and  arteries  called  the  venae  vorticosse. 


THE   SENSE   OF    SIGHT. 


239 


4.  The  lamina  vitrea,  or  internal  limiting  membrane. 

The  choroid  with  its  contained  blood-vessels  bears  an  important 
relation  to  the  nutrition  of  the  eye ;  it  provides  for  the  blood-supply 
and  for  drainage  from  the  body  of  the  eye,  and  presents  a  uniform 
and  high  temperature  to  the  retina. 

The  iris  is  the  circular  variously  colored  membrane  placed  in  the 
anterior  portion  of  the  eye  just  behind  the  cornea.  It  is  perforated 
a  little  to  the  nasal  side  of  the  center  by  a  circular  opening,  the  pupil. 
The  outer  or  circumferential  border  is  connected  with  the  cornea, 


FIG.  30. — SCLEROTIC  COAT  REMOVED  TO  SHOW  CHOROID  CILIARY  MUSCLE,  AND 
NERVES. —  (From  Holdcn's  "Anatomy.") 

i.  Sclerotic  coat.     b.  Veins  of  the  choroid.     c.  Ciliary  nerves,     d.  Veins  of 
the  choroid.     e.  Ciliary  muscle,     f.  Iris. 

ciliary  muscle,  and  ciliary  processes ;  the  free  inner  edge  forms  the 
boundary  of  the  pupil,  the  size  of  which  is  constantly  changing. 
The  framework  of  the  iris  is  composed  of  connective-tissue  blood- 
vessels, muscle-fibers  and  pigmented  connective-tissue  corpuscles. 
The  anterior  surface  is  covered  with  a  layer  of  epithelial  cells  con- 
tinuous with  those  covering  the  posterior  surface  of  the  cornea ;  the 
posterior  surface  is  lined  by  a  limiting  membrane  bearing  pigment 
epithelial  cells  continuous  with  those  of  the  choroid.  The  various 
colors  which  the  iris  assumes  in  different  individuals  depend  upon 
the  quantity  and  disposition  of  the  pigment  granules. 


240  HUMAN   PHYSIOLOGY. 

The  muscle-fibers  of  the  iris,  which  are  of  the  non-striated  variety, 
are  arranged  in  two  sets — the  sphincter  and  dilatator. 

The  sphincter  pupilla  is  a  circular,  flat  band  of  muscle-fibers  sur- 
rounding the  pupil  close  to  its  posterior  surface  ;  by  its  contraction 
and  relaxation  the  pupil  is  diminished  or  increased  in  size.  The 
dilatator  pupillce  consists  of  a  thin  layer  of  fibers  arranged  in  a 
radiate  manner;  at  the  margin  of  the  pupil  they  blend  with  those 
of  the  sphincter  muscle,  while  at  the  outer  border  they  arch  to  form  a 
circular  muscular  layer. 

The  ciliary  muscle  is  a  gray,  circular  band,  consisting  of  unstriped 
muscle-fibers  about  T^  of  an  inch  long  running  from  before  back- 
ward. It  is  attached  anteriorly  to  the  inner  surface  of  the  sclerotic 
and  cornea,  and  posteriorly  to  the  choroid  coat  opposite  the  ciliary 
processes.  At  the  anterior  border  of  the  radiating  fibers  and  internally 
are  found  bundles  of  circular  muscle-fibers,  constituting  the  annular 
muscle  of  Miiller.  The  ciliary  muscle  thus  consists  of  two  sets,  of 
fibers,  a  radiating  and  a  circular,  both  of  which  are  concerned  in 
effecting  a  change  in  the  convexity  of  the  lens  in  the  accommodation 
of  the  eye  to  near  vision. 

The  retina  forms  the  internal  coat  of  the  eye.  In  the  fresh 
state  it  is  a  delicate,  transparent  membrane  of  a  pink  color,  but  after 
death  soon  becomes  opaque ;  it  extends  forward  almost  to  the  ciliary 
processes,  where  it  terminates  in  an  indented  border,  the  ora  serrata. 
In  the  posterior  part  of  the  retina,  at  a  point  corresponding  to  the 
axis  of  vision,  is  a  yellow  spot,  the  macula  lutea,  which  is  somewhat 
oval  in  shape  and  tinged  with  yellow  pigment.  It  presents  in  its 
center  a  depression,  the  fovea  centralis,  corresponding  to  a  decrease 
in  thickness  of  the  retina ;  about  -fa  of  an  inch  to  the  inner  side  of 
the  macula  is  the  point  of  entrance  of  the  optic  nerves.  The  arteria 
centralis  retina  pierces  the  optic  nerve  near  the  sclerotic,  runs  for- 
ward in  its  substance,  and  is  distributed  in  the  retina  as  far  forward 
as  the  ciliary  processes. 

The  retina  is  remarkably  complex,  consisting  of  ten  distinct  layers, 
from  within  outward,  supported  by  connective  tissue.  These  are  as 
follows — viz. : 

1.  Membrana  limitans  interna. 

2.  Fibers  of  optic  nerve. 

3.  Layers  of  ganglionic  corpuscles. 

4.  Molecular  layer. 

5.  Internal  granular  layer. 


THE   SENSE  OF   SIGHT.  241 

6.  Molecular  layer. 

7.  External  granular  layer. 

8.  Membrana  limitans  externa. 

9.  Jacobson's  membrane,   or  layer  of  rods   and  cones. 
10.  The  layer  of  pigment  cells. 

The  most  important  of  these,  however,  is  the  layer  of  rods  and 
cones  in  the  external  portion  of  the  retina.  The  rods  are  straight, 
elongated  cylinders  extending  through  the  entire  thickness  of  Jacob- 
son's  membrane.  They  consist  of  an  external  portion,  which  is 
clear,  homogeneous,  and  highly  refracting,  and  of  an  internal  por- 
tion, which  is  slightly  granular  and  less  refractive ;  the  outer  end 
of  each  rod  is  in  direct  contact  with  the  pigment  epithelium  lining 
the  choroid,  while  the  inner  end,  tapering  to  a  fine  thread,  pierces 
the  external  limiting  membrane  and  passes  into  the  external  granular 
layer.  The  cones  consist  also  of  two  portions,  the  inner  of  which 
is  somewhat  thicker  than  the  rod  and  rests  upon  the  limiting  mem- 
brane ;  the  outer  portion  tapers  to  a  fine  point,  which  is  known  as  the 
cone-style.  The  cones,  as  a  rule,  are  somewhat  shorter  than  the 
rods.  The  proportion  of  rods  to  cones  varies  in  different  parts  of 
the  retina,  though  there  are  on  an  average  about  fourteen  rods  to  one 
cone.  In  the  macula  lutea,  where  vision  is  most  acute,  the  rods 
are  almost  entirely  absent,  cones  alone  being  present.  All  the 
retinal  elements  at  this  point  are  changed.  The  nerve-fiber  layer 
is  absent,  the  axis-cylinders  radiating  in  such  a  manner  as  to  leave 
the  spot  free  from  their  covering.  The  remaining  layers  are  all  thinned 
and  the  stroma  is  reduced  to  a  minimum.  The  optic  nerve,  after 
passing  forward  from  the  brain,  penetrates  in  succession  the  sclerotic, 
choroid,  and  retina ;  the  nerve-fibers  then  spread  out  over  the  anterior 
surface  of  the  retina  and  become  connected  with  the  large  ganglionic 
cells,  the  third  layer  of  the  retina. 

The  number  of  optic  nerve-fibers  in  the  retina  is  estimated  to  be 
about  800,000,  and  for  each  fiber  there  are  about  seven  cones,  one 
hundred  rods,  and  seven  pigment  cells.  The  points  of  the  rods  and 
cones  are  directed  toward  the  choroid,  or  away  from  the  entering 
light,  and  dip  into  the  pigment  layer.  They,  with  the  pigment  layer, 
are  the  intermediating  elements  in  the  change  of  the  ethereal  vibra- 
tions into  nerve  force ;  out  of  these  nerve  vibrations  the  brain 
fashions  the  sensations  of  light,  form,  and  color. 

The  vitreous  humor,  which  supports  the  retina,  is  the  largest  of  the 
17 


242  HUMAN   PHYSIOLOGY. 

refracting  media;  it  is  globular  in  form  and  constitutes  about  four 
fifths  of  the  ball ;  it  is  hollowed  out  anteriorly  for  the  reception  of 
the  crystalline  lens.  The  outer  surface  of  the  vitreous  is  covered  by 
a  delicate,  transparent  membrane,  termed  the  hyaloid  membrane, 
which  serves  to  maintain  its  globular  form. 

The  aqueous  humor,  found  in  the  anterior  chamber  of  the  eye, 
is  a  clear  alkaline  fluid,  having  a  specific  gravity  of  1003-1009. 
It  is  secreted  most  probably  by  the  blood-vessels  of  the  iris  and  ciliary 
processes.  It  passes  from  the  interior  of  the  eye,  through  the  canal 
of  Schlemm  and  the  meshes  at  the  base  of  the  iris,  into  the  anterior 
circular  vein. 

The  crystalline  lens,  inclosed  within  its  capsule,  is  a  transparent 
bicovex  body,  situated  just  behind  the  iris  and  resting  in  the  depres- 
sion in  the  anterior  part  of  the  vitreous.  The  two  convexities  are 
not  quite  alike,  the  curvature  of  the  posterior  surface  being  slightly 
greater  than  that  of  the  anterior.  The  lens  measures  about  J^  of  an 
inch  in  the  transverse  diameter  and  Ys  of  an  inch  in  the  antero- 
posterior  diameter. 

The  suspensory  ligament,  by  which  the  lens  is  held  in  position,  is 
a  firm,  transparent  membrane,  united  to  the  ciliary  processes.  A 
short  distance  beyond  its  origin  it  splits  into  two  layers,  the  anterior 
of  which  is  inserted  into  the  capsule  of  the  lens  and  blends  with  it ; 
the  posterior,  passing  inward  behind  the  lens,  becomes  united  to 
its  capsule.  The  anterior  layer  presents  a  series  of  foldings,  zone  of 
Zinn,  which  are  inserted  into  the  intervals  of  the  folds  of  the  ciliary 
processes.  The  triangular  space  between  the  two  layers  is  the  canal 
of  Petit. 

Blood-vessels  and  Nerves. — The  structures  composing  the  eyeball 
are  supplied  with  blood  by  the  long  and  short  ciliary  arteries,  branches 
of  the  ophthalmic;  they  pierce  the  sclerotic  at  various  points  and 
are  ultimately  distributed  to  all  tissues  within  the  ball. 

The  nerve-supply  comes  largely  from  the  ophthalmic  or  ciliary 
ganglion.  This  is  a  small  body,  situated  in  the  posterior  part  of  the 
orbit ;  it  receives  motor  fibers  from  a  branch  of  the  motor  oculi,  or 
third  nerve ;  a  sensory  branch  from  the  ophthalmic  division  of  the 
fifth  nerve,  and  fibers  from  the  cavernous  plexus  of  the  sympathetic. 
From  the  anterior  border  of  the  ganglion  proceed  the  ciliary  nerves, 
which,  entering  the  eyeball,  endow  its  structures  with  motion  and 
sensation. 


THE   SENSE   OF    SIGHT. 


243 


The  Eyeball  a  Living  Camera  Obscura. — The  eyeball  may  be  com- 
pared in  a  general  way  to  a  camera  obscura.  The  anatomic  arrange- 
ment of  its  structures  reveals  many  points  of  similarity.  The 
sclerotic  and  choroid  may  be  compared  with  the  walls  of  the  chamber ; 
the  combined  refractive  media,  cornea,  aqueous  humor,  lens,  and 
vitreous  humor,  to  the  lens  for  focusing  the  rays  of  light ;  the  retina, 
to  the  sensitive  plate  receiving  the  image  formed  at  the  focal  point ; 
the  iris,  to  the  diaphragm,  which,  by  cutting  off  the  marginal  rays,  pre- 
vents spheric  aberration  and  at  the  same  time  regulates  the  amount 


d 


FIG.   31. — DIAGRAM  OF  A  VERTICAL   SECTION   OF  THE   EYE. —  (From  Holden's 
"Anatomy.") 

i.  Anterior  chamber  filled  with  aqueous  humor.  2.  Posterior  chamber.  3. 
Canal  of  Petit,  a.  Hyaloid  membrane.  b.  Retina  (dotted  line).  c. 
Choroid  coat  (black  line),  d.  Sclerotic  coat.  e.  Cornea,  f.  Iris.  g. 
Ciliary  processes,  h.  Canal  of  Schlemm  or  Fontana.  i.  Ciliary  muscle. 

of  light  entering  the  eye ;  the  ciliary  muscle,  to  the  adjusting  screw,  by 
which  distinct  images  are  thrown  upon  the  retina  in  spite  of  varying 
distances  of  the  object  from  which  the  light  rays  emanate.  The 
structures  just  enumerated  are  those  essential  for  normal  vision. 

The  relationship   of  the   various   structures   composing  the   eyeball 
is  shown  in  Figure  31. 

The  dioptric  or  refracting  apparatus,  by  which  the  rays  of  light 
entering  the  eye  are  so  manipulated  as  to  produce  an  image  on  the 


244 


HUMAN   PHYSIOLOGY. 


retina,  consists   of  the  cornea,   aqueous  humor,   crystalline  lens,   and 
vitreous   humor.     A   ray   of  light  in   passing  through   each   of  these 
media   will   undergo   refraction   at   their   surfaces   and   ultimately   be 
brought  to  a  focus  at  the  retina.     Inasmuch  as  the  two  surfaces  of 
the  cornea  are  parallel  and  its  refractive  power  practically  the  same 
as   the   aqueous   humor,   the   media   may   be   reduced   to   three — viz. : 
i.  Cornea  and  aqueous  humor. 
2..  The  lens. 
3.  The  vitreous  humor. 

The  refracting  surfaces  may  also  be  reduced  to  three — viz. : 

1.  Anterior  surface  of  the  cornea. 

2.  Anterior  surface  of  lens. 

3.  Posterior  surface  of  lens. 

The  refraction  effected  by  the  cornea  is  very  great,  owing  to  the 
passage  of  the  light  from  the  air  into  a  comparatively  dense  medium, 
and  is  sufficient  of  itself  to,  bring  parallel  rays  of  light  to  a  focus 


FIG.  32. — DIAGRAM  SHOWING  THE  COURSE  OF  PARALLEL  RAYS  OF  LIGHT  FROM 
A,  IN  THEIR  PASSAGE  THROUGH  A  BICONVEX  LENS,  L,  IN  WHICH  THEY  ARE 
so  REFRACTED  AS  TO  BIND  TOWARD  AND  COME  IN  A  Focus  AT  A  POINT,  F. 
—  (From  Yeo's  "Text-book  of  Physiology/') 


pIG>  33> — DIAGRAM  SHOWING  THE  COURSE  OF  DIVERGING  RAYS  WHICH  ARE  BENT 
TO  A  POINT  FURTHER  FROM  THE  LENS  THAN  THE  PARALLEL  RAYS  IN  PRE- 
CEDING FIGURE. —  (Yeo's  "Text-book  of  Physiology.") 

about  ten  millimeters  behind  the  retina.  This  would  be  the  condi- 
tion in  an  eye  in  which  the  lens  was  congenitally  absent.  Perfect 
vision  requires,  however,  that  the  convergence  of  the  light  shall  be 
great  enough  to  allow  the  image  to  fall  upon  the  retina.  This  is 


THE  SENSE  OF   SIGHT.  245 

accomplished  by  the  crystalline  lens,  a  body  denser  than  the  cornea 
and  possessing  a  higher  refractive  power.  The  manner  in  which  a 
biconvex  lens  focuses  both  parallel  and  divergent  rays  is  shown  in 
figures  32  and  33. 

The  function  of  the  crystalline  lens,  therefore,  is  to  focus  the  rays 
of  light  with  the  formation  of  an  image  on  the  retina. 

The  retinal  image  corresponds  in  all  respects  to  the  object  from 
which  the  light  proceeds.  The  existence  of  this  image  can  be  demon- 
strated by  removing  from  the  eye  of  a  recently  killed  animal  a  cir- 
cular portion  of*  the  sclerotic  and  choroid  posteriorly,  and  then 
placing  at  the  proper  distance  in  front  of  the  cornea  a  lighted  candle ; 
an  inverted  image  of  the  candle  will  be  seen  upon  the  retina.  The 
size  of  the  retinal  image  depends  upon  the  visual  angle,  which  in 
turn  depends  upon  the  size  of  the  object  and  its  distance  from  the 
eye.  At  a  distance  of  15.2596  meters  the  image  of  an  object  one  meter 
high  would  be  one  millimeter,  or  a  thousand  times  smaller  than 
the  object. 

Accommodation. — By  accommodation  is  understood  the  power 
which  the  eye  possesses  of  adjusting  itself  to  vision  at  different 
distances.  In  a  normal  or  emmetropic  eye  parallel  rays  of  light 
are  brought  to  a  focus  on  the  retina  ;  but  divergent  rays — that  is, 
rays  coming  from  a  near  luminous  point — will  be  brought  to  a  focus 
behind  the  retina,  provided  the  refractive  media  remains  the  same ; 
as  a  result,  vision  would  be  indistinct,  from  the  formation  of 
diffusion  circles.  It  is  impossible  to  see  distinctly,  therefore,  a 
near  and  a  distant  object  at  the  same  time.  We  must  alternately 
direct  the  vision  from  one  to  the  other.  A  normal  eye  does  not 
require  adjusting  for  parallel  rays  ;  but  for  divergent  rays  a  change 
in  the  eye  is  necessitated ;  this  is  termed  accommodation.  In  the 
accommodation  for  near  vision  the  lens  becomes  more  convex,  par- 
ticularly on  its  anterior  surface.  The  increase  in  convexity  aug- 
ments its  refractive  power ;  the  greater  the  degree  of  divergence  of 
the  rays  previous  to  entering  the  eye,  the  greater  the  increase  of  con- 
vexity of  the  lens  and  convergence  of  the  rays  after  passing  through 
it.  By  this  alteration  in  the  shape  of  the  lens  we  are  enabled  to 
focus  light  rays  coming  from,  and  to  see  distinctly,  near  as  well  as 
distant  objects. 

Function  of  the  Ciliary  Muscle. — Though  it  is  admitted  that  the 
change  in  the  convexity  of  the  lens  is  caused  by  the  contraction  of 


246  HUMAN   PHYSIOLOGY. 

the  ciliary  muscle  and  the  relaxation  of  the  suspensory  ligament, 
the  exact  manner  in  which  it  does  so  is  not  understood.  When  the 
eye  is  in  repose,  as  in  distant  vision,  the  suspensory  ligament  is 
tense,  and  the  lens  possesses  that  degree  of  curvature  necessary  for 
focusing  parallel  rays.  In  the  voluntary  efforts  to  accommodate  the 
eye  for  near  vision,  the  ciliary  muscle  contracts,  the  suspensory  liga- 
ment relaxes,  and  the  lens,  inherently  elastic,  bulges  forward  and 
once  again  focuses  the  rays  upon  the  retina.  It  is,  therefore,  termed 
the  muscle  of  accommodation,  and  by  its  alternate  contraction  and 
relaxation  the  lens  is  rendered  more  or  less  convex^  according  to  the 
requirements  for  near  and  distant  vision. 

Range  of  Accommodation. — Parallel  rays  coming  from  a  luminous 
point  distant  not  less  than  200  feet  do  not  require  adjustment; 
from  this  point  up  to  infinity  no  accommodation  is  required  for  per- 
fect vision.  This  is  termed  the  punctum  remotum,  and  indicates  the 
distance  to  which  an  object  may  be  removed  and  yet  distinctly  seen. 
If  the  object  be  brought  nearer  to  the  eye  than  200  feet,  the  accom- 
modative power  must  come  into  play;  the  nearer  the  object,  the  more 
energetic  must  be  the  contraction  of  the  ciliary  muscle  and  the  cbn- 
sequent  increase  in  the  convexity  of  the  lens.  At  a  distance  of  five 
inches,  however,  the  power  of  accommodation  reaches  its  maximum ; 
this  is  termed  the  punctum  proximum,  and  indicates  the  nearest  point 
at  which  an  object  may  be  seen  distinctly.  The  distance  between 
these  two  points  is  the  range  of  accommodation. 

Optic  Defects. — Astigmatism  is  a  condition  of  the  eye  which 
prevents  vertical  and  horizontal  lines  from  being  focused  at  the  same 
time,  and  is  due  to  a  greater  curvature  of  the  cornea  in  one  meridian 
than  in  another. 

Spheric  aberration  is  a  condition  in  which  there  is  an  indistinctness 
of  an  image  from  the  unequal  refraction  of  the  rays  of  light  passing 
through  the  circumference  and  the  center  of  the  lens ;  it  is  corrected 
mainly  by  the  iris,  which  cuts  off  the  marginal  rays,  and  transmits 
only  those  passing  through  the  center. 

Chromatic  aberration  is  a  condition  in  which  the  image  is  sur- 
rounded by  a  colored  margin,  from  the  decomposition  of  the  rays 
of  light  into  their  elementary  parts. 

Myopia,  or  shortsightness,  is  caused  by  an  abnormal  increase  in 
the  anteroposterior  diameter  of  the  eyeball,  or  by  a  hypernormal 
refracting  power  of  the  lens.  It  is  generally  due  to  the  first  cause; 


THE  SENSE  OF   SIGHT.  247 

the  lens,  being  too  far  removed  from  the  retina,  forms  the  image 
in  front  of  it,  and  the  perception  becomes  dim  and  blurred.  Concave 
glasses  correct  this  defect  by  preventing  the  rays  from  converging 
too  soon. 

Hypermetropia,  or  longsightness,  is  caused  by  a  shortening  of  the 
anteroposterior  diameter  or  by  a  subnormal  refractive  power  of  the 
lens ;  the  focus  of  the  rays  of  light  would,  therefore,  be  behind  the 
retina.  Convex  glasses  correct  this  defect  by  converging  the  rays 
of  light  more  anteriorly. 

Presbyopia  is  a  loss  of  the  power  of  accommodation  of  the  eye 
to  near  objects,  and  usually  occurs  between  the  ages  of  forty  and 
sixty ;  it  is  remedied  by  the  use  of  convex  glasses. 

The  Iris. — The  iris  plays  the  part  of  a  diaphragm,  and  by  means 
of  its  central  aperture  the  pupil  regulates  the  quantity  of  light 
entering  the  interior  of  the  eye ;  by  preventing  rays  from  passing 
through  the  margin  of  the  lens  it  diminishes  spheric  aberration.  The 
size  of  the  pupil  depends  upon  the  relative  degree  of  contraction  of 
the  circular  and  radiating  fibers  ;  the  variations  in  size  of  the  pupil 
from  variations  in  the  degree  of  contraction  depend  upon  different 
intensities  of  light.  If  the  light  be  intense,  the  circular  fibers 
contract,  and  diminish  the  size  of  the  pupil ;  if  the  light  diminishes 
in  intensity,  the  circular  fibers  relax  and  the  pupil  enlarges. 

Point  of  Most  Distinct  Vision. — While  all  portions  of  the  retina 
are  sensitive  to  light,  their  sensibility  varies  within  wide  limits.  At 
the  macula  lutea,  and  more  especially  in  its  most  central  depression, 
the  fovea,  where  the  retinal  elements  are  reduced  practically  to  the 
layer  of  rods  and  cones,  the  sensibility  reaches  its  maximum.  It  is 
at  this  point  that  the  image  is  found  when  vision  is  most  distinct. 
The  macula  and  fovea  are  always  in  the  line  of  direct  vision.  From 
the  macula  toward  the  periphery  of  the  retina  there  is  a  gradual 
diminution  in  sensibility,  and  a  corresponding  decline  in  the  dis- 
tinctness of  vision.  In  those  portions  of  the  retina  lying  outside  the 
macula,  the  indistinctness  of  vision  depends  not  only  on  diminished 
sensibility,  but  also  upon  inaccurate  focusing  of  the  rays. 

Blind  Spot. — Although  the  optic  nerve  transmits  the  impulses 
excited  in  the  retina  by  the  ethereal  vibration,  the  nerve-fibers  them- 
selves are  insensitive  to  light.  At  the  point  of  entrance  of  the  optic 
nerve,  owing  to  the  absence  of  the  rods  and  cones,  the  rays  of  light 


248  HUMAN   PHYSIOLOGY. 

make  no  impression.  This  is  the  blind  spot.  As  this  spot  is  not 
in  the  line  of  vision,  no  dark  point  is  ordinarily  observed  in  the 
field  of  vision — the  circular  space  before  a  fixed  eye  within  which 
reflections  of  objects  are  perceptible. 

The  rods  and  cones  are  the  most  sensitive  portions  of  the  retina. 
A  ray  of  light  entering  the  eye  passes  entirely  through  the  various 
layers  of  the  retina,  and  is  arrested  only  upon  reaching  the  pig- 
mentary epithelium  in  which  the  rods  and  cones  are  embedded.  As 
to  the  manner  in  which  the  objective  stimuli — light  and  color,  so 
called — are  transformed  into  nerve  impulses,  but  little  is  known. 
It  is  probable  that  the  ethereal  vibrations  are  transformed  into 
heat,  which  excites  the  rods  and  cones.  These,  acting  as  highly 
specialized  end  organs  of  the  optic  nerve,  start  the  impulses  on  their 
way  to  the  brain,  where  the  seeing  process  takes  place.  As  to  the 
relative  function  of  the  rods  and  cones,  it  has  been  suggested,  from 
the  study  of  the  facts  of  comparative  anatomy,  that  the  rods  are  im- 
pressed only  by  differences  in  the  intensity  of  light,  while  the  cones, 
in  addition,  are  impressed  by  qualitative  differences  or  color. 

Accessory  Structures. — The  muscles  which  move  the  eyeball  are 
six  in  number — the  superior  and  inferior  recti,  the  external  and  in- 
ternal recti,  the  superior  and  inferior  oblique  muscles.  The  four 
recti  muscles,  arising  from  the  apex  of  the  orbit  pass  forward  and 
are  inserted  into  the  sides  of  the  sclerotic  coat ;  the  superior  and 
inferior  muscles  rotate  the  eye  around  a  horizontal  axis ;  the  ex- 
ternal and  internal  rotate  it  around  a  vertical  axis. 

The  superior  oblique  muscle,  having  the  same  origin,  passes  for- 
ward to  the  inner  and  upper  angle  of  the  orbital  cavity,  where  its 
tendon  passes  through  a  cartilaginous  pulley ;  it  is  then  reflected 
backward  and  inserted  into  the  sclerotic  just  behind  the  transverse 
diameter.  Its  function  is  to  rotate  the  eyeball  in  such  a  manner  as 
to  direct  the  pupil  downzvard  and  outward. 

The  inferior  oblique  muscle  arises  at  the  inner  angle  of  the  orbit, 
and  then  passes  outward  and  backward,  to  be  inserted  into  the 
sclerotic.  Its  function  is  to  rotate  the  eyeball  and  to  direct  the 
pupil  upward  and  outward. 

By  the  associated  action  of  all  these  muscles,  the  eyeball  is  capable 
of  performing  all  the  varied  and  complex  movements  necessary  for 
distinct  vision. 

The   eyelids,  bordered   with   short,   stiff   hairs,   shade   the   eye   and 


THE   SENSE  OF    HEARING.  249 

protect  it  from  injury.  On  the  posterior  surface,  just  beneath  the 
conjunctiva,  are  the  Meibomian  glands,  which  secrete  an  oily  fluid; 
this  covers  the  edge  of  the  lids,  and  prevents  the  tears  from  flowing 
over  the  cheek. 

The  lacrymal  glands  are  ovoid  in  shape,  and  are  situated  at  the 
upper  and  outer  part  of  the  orbital  cavity  ;  they  open  by  from  six 
to  eight  ducts  at  the  outer  portion  of  the  upper  lids. 

The  tears,  secreted  by  the  lacrymal  glands,  are  distributed  over 
the  cornea  by  the  lids  during  the  act  of  winking,  and  keep  it  moist 
and  free  from  dust.  The  excess  of  tears  passes  into  the  lacrymal 
ducts,  which  begin  by  two  minute  orifices,  one  on  each  lid,  at  the 
inner  canthus.  They  conduct  the  tears  into  the  nasal  duct,  and  so 
into  the  nose. 


THE    SENSE    OF    HEARING. 

The  ear,  or  organ  of  hearing,  is  lodged  within  the  petrous  portion 
of  the  temporal  bone.  It  may  be,  for  convenience  of  description, 
divided  into  three  portions — viz. : 

1.  The  external  ear. 

2.  The  middle  ear. 

3.  The  internal  ear  or  labyrinth. 

The  external  ear  consists  of  the  pinna,  or  auricle,  and  the  ex- 
ternal auditory  canal.  The  pinna  consists  of  a  thin  layer  of  cartilage, 
presenting  a  series  of  elevations  and  depressions  ;  it  is  attached  by 
fibrous  tissue  to  the  outer  bony  edge  of  the  auditory  canal ;  it  is 
covered  by  a  layer  of  integument  continuous  with  that  covering  the 
side  of  the  head.  The  general  shape  of  the  pinna  is  concave,  and 
presents,  a  little  below  the  center,  a  deep  depression — the  concha. 
The  external  auditory  canal  extends  from  the  concha  inward  for  a 
distance  of  about  i*4  inches.  It  is  directed  somewhat  forward  and 
upward,  passing  over  a  convexity  of  bone,  and  then  dips  downward 
to  its  termination ;  it  is  composed  of  both  bone  and  cartilage,  and 
is  lined  by  a  reflection  of  the  skin  covering  the  pinna.  At  the  ex- 
ternal portion  of  the  canal  the  skin  contains  a  number  of  tubular 
glands, — the'  ceruminous  glands, — which  in  their  conformation  re- 
semble the  perspiratory  glands.  They  secrete  the  cerumen,  or  ear-wax. 

The  middle  ear,  or  tympanum,  is  an  irregularly  shaped  cavity 
hollowed  out  of  the  temporal  bone  and  situated  between  the  external 


250  HUMAN   PHYSIOLOGY. 

ear  and  the  labyrinth.  It  is  narrow  from  side  to  side,  but  rela- 
tively long  in  its  vertical  and  anteroposterior  diameters ;  it  is  sep- 
arated from  the  external  auditory  canal  by  a  membrane — the  mem- 
brana  tympani ;  from  the  internal  ear  it  is  separated  by  an  osseo- 
membranous  partition,  which  forms  a  common  wall  for  both  cavities. 
The  middle  ear  communicates  posteriorly  with  the  mastoid  cells ; 
anteriorly  with  the  nasopharynx,  by  means  of  the  Eustachian  tube. 
The  interior  of  this  cavity  is  lined  by  mucous  membrane  continuous 
with  that  lining  the  pharynx. 

The  membrana  tympani  is  a  thin,  translucent,  nearly  circular  mem- 
brane, measuring  about  ^i  of  an  inch  in  diameter,  placed  at  the  inner 
termination  of  the  external  auditory  canal.  The  membrane  is  in- 
closed within  a  ring  of  bone,  which  in  the  fetal  condition  can  be 
easily  removed,  but  in  the  adult  condition  becomes  consolidated  with 
the  surrounding  bone.  The  membrana  tympani  consists  primarily  of 
a  layer  of  fibrous  tissue,  arranged  both  circularly  and  radially,  and 
forms  the  membrana  propria;  externally  it  is  covered  by  a  thin  layer 
of  skin  continuous  with  that  lining  the  auditory  canal ;  internally 
it  is  covered  by  a  thin  mucous  membrane.  The  tympanic  membrane 
is  placed  obliquely  at  the  bottom  of  the  auditory  canal,  inclining  at 
an  angle  of  forty-five  degrees,  being  directed  from  behind  and  above 
downward  and  inward.  On  its  external  surface  this  membrane  pre- 
sents a  funnel-shaped  depression,  the  sides  of  which  are  somewhat 
convex. 

The  Ear  Bones. — Running  across  the  tympanic  cavity  and  forming 
an  irregular  line  of  joined  levers  is  a  chain  of  bones  which  articulate 
with  one  another  at  their  extremities.  They  are  known  as  the 
malleus,  incus,  and  stapes. 

The  form  and  position  of  these  bones  are  shown  in  figure  34. 

The  malleus  consists  of  a  head,  neck,  and  handle,  of  which  the 
latter  is  attached  to  the  inner  surface  of  the  membrana  tympani ;  the 
incus,  or  anvil  bone,  presents  a  concave,  articular  surface,  which  re- 
ceives the  head  of  the  malleus  ;  the  stapes,  or  stirrup  bone,  articulates 
externally  with  the  long  process  of  the  incus,  and  internally,  by  its 
oval  base,  with  the  edges  of  the  foramen  ovale. 

The  tensor  tympani  muscle  consists  of  a  fleshy,  tapering  portion, 
Y*  of  an  inch  in  length,  which  terminates  in  a  slender  tendon ;  it 
arises  from  the  cartilaginous  portion  of  the  Eustachian  tube  and  the 
adjacent  surface  of  the  sphenoid  bone.  From  this  origin  the  muscle 


THE   SENSE   OF   HEARING. 


251 


passes  nearly  horizontally  backward  to  the  tympanic  cavity ;  just 
opposite  to  the  fenestra  ovalis  its  tendon  bends  at  a  right  angle 
over  the  processus  cochleariformis,  and  then  passes  outward  across 
the  cavity,  to  be  inserted  into  the  angle  of  the  malleus  near  the  neck. 
The  stapedius  muscle  emerges  from  the  cavity  of  a  pyramid  of 
bone  projecting  from  the  posterior  wall  of  the  tympanum  ;  the  tendon 
passes  forward,  and  is  inserted  into  the  neck  of  the  stapes  bone,  pos- 
teriorly, near  its  point  of  articulation  with  the  incus. 


A.G. 


AUDITORY  OSSICLES  (LEFT)  MAGNIFIED 


The  laxator  tympani  muscle,  so  called,  is  now  generally  regarded 
as  being  ligamentous  in  nature,  and  not  muscular. 

The  Eustachian  tube,  by  means   of  which   a  free   communication 
is    established   between    the    middle    ear   and   the   pharynx,    is   partly 


252  HUMAN    PHYSIOLOGY. 

bony  and  partly  cartilaginous  in  structure.  It  measures  about  1^/2 
inches  in  length ;  commencing  at  its  opening  into  the  nasopharynx, 
it  passes  upward  and  outward  to  the  spine  of  the  sphenoid  bone,  at 
which  point  it  becomes  somewhat  contracted;  the  tube  then  dilates 
as  it  passes  backward  into  the  middle-ear  cavity ;  it  is  lined  by 
mucous  membrane,  which  is  continued  into  the  middle  ear  and  mas- 
toid  cells. 

The  function  of  the  ear,  as  a  whole,  is  the  reception  and  trans- 
mission of  aerial  vibrations  to  the  terminal  organs  concealed  within 
the  internal  ear,  and  which  are  connected  with  the  auditory  nerve- 
fibers.  The  excitation  of  these  end  organs  caused  by  the  impact  of 
the  vibrations  arouses  in  the  auditory  nerve  impulses  which  are 
then  transmitted  to  the  brain,  where  the  hearing  process  takes  place. 
In  order  to  appreciate  the  functions  of  the  individual  parts  of  the 
ear,  a  few  of  the  characteristics  of  sound  waves  must  be  kept  in 
mind. 

Sound  Waves. — All  sounds  are  caused  by  vibrations  in  the  atmo- 
sphere which  have  been  communicated  to  it  by  vibrating  elastic 
bodies,  such  as  membranes,  strings,  rods,  etc.  These  vibrating  bodies 
produce  in  the  air  a  to-and-fro  movement  of  its  particles,  resulting 
in  a  series  of  alternate  condensations  and  rarefactions,  which  are 
propagated  in  all  directions.  A  complete  oscillation  of  a  particle 
of  air  forward  and  backward  constitutes  a  sound  wave.  Musical 
sounds  are  caused  by  a  succession  of  regular  waves,  which  follow 
one  another  with  a  certain  rapidity.  Noises  are  caused  by  the  impact 
of  a  series  of  irregular  waves. 

All  sound  waves  possess  intensity,  pitch,  and  equality.  The  in- 
tensity, or  loudness,  of  a  sound  depends  upon  the  amplitude  of  its 
vibrations  or  on  the  extent  of  its  excursion.  The  pitch  depends  upon 
the  number  of  vibrations  which  affect  the  auditory  nerve  in  a  second 
of  time  ;  the  pitch  of  the  note  C,  the  first  below  the  leger  line  of  the 
musical  scale,  is  caused  by  256  vibrations  a  second  ;  the  pitch  of  the 
same  note  an  octave  higher  is  caused  by  512  vibrations  a  second.  If  the 
vibrations  are  too  few  a  second,  they  fail  to  be  perceived  as  a  con- 
tinuous sound ;  the  minimum  number  of  vibrations  capable  of  pro- 
ducing a  sound  has  been  fixed  at  sixteen  a  second;  the  highest 
pitched  musical  note  capable  of  being  heard  has  been  shown  to  be 
due  to  38,000  vibrations  a  second.  In  the  ascent  of  the  musical 
scales  there  is,  therefore,  a  gradual  increase  in  the  number  of  vibra- 


THE   SENSE  OF   HEARING.  253 

tions  and  a  gradual  increase  in  the  pitch  of  the  sounds.  Between 
the  two  extreme  limits  lies  the  range  of  audibility,  which  embraces 
eleven  octaves,  of  which  seven  are  employed  in  the  musical  scale. 
The  quality  of  sound  depends  upon  a  combination  of  the  funda- 
mental vibration  with  certain  secondary  vibrations  of  subdivisions 
of  the  vibrating  body.  These  so-called  over-tones  vary  in  intensity 
and  pitch,  and  by  modifying  the  form  of  the  primary  wave  produce 
that  which  is  termed  the  quality  of  sound. 

Function  of  the  Pinna  and  External  Auditory  Canal. — In  those 
animals  possessing  movable  ears  the  pinna  plays  an  important  part 
in  the  collection  of  sound  waves.  In  man,  in  whom  the  capability  of 
moving  the  pinna  has  been  lost,  it  is  doubtful  if  it  is  at  all  necessary 
for  hearing.  Nevertheless  an  individual  with  dull  hearing  may 
have  the  perception  of  sound  increased  by  placing  the  pinna  at  an 
angle  of  45  degrees  to  the  side  of  the  head.  The  external  auditory 
canal  transmits  the  sonorous  vibrations  to  the  tympanic  membrane. 
Owing  to  the  obliquity  of  this  canal  it  has  been  supposed  that  the 
waves,  concentrated  at  the  concha,  undergo  a  series  of  reflections 
on  their  way  to  the  tympanic  membrane,  and,  owing  to  the  position 
of  this  membrane,  strike  it  almost  perpendicularly. 

Function  of  the  Tympanic  Membrane. — The  function  of  the  tym- 
panic membrane  appears  to  be  the  reception  of  sound  vibration^  by 
being  thrown  by  them  into  reciprocal  vibrations  which  correspond 
in  intensity  and  amplitude.  That  this  membrane  actually  reproduces 
all  vibrations  within  the  range  of  audibility  has  been  experimentally 
demonstrated.  The  membrane  not  being  fixed,  so  far  as  its  tension 
is  concerned,  does  not  possess  a  fixed  fundamental  note,  like  a  sta- 
tionary fixed  membrane,  and  is,  therefore,  just  as  well  adapted  for 
the  reception  of  one  set  of  vibrations  as  for  another.  This  is  made 
possible  by  variations  in  its  tension  in  accordance  with  the  pitch  of 
the  sounds.  In  the  absence  of  all  sound  the  membrane  is  in  a 
condition  of  relaxation ;  with  the  advent  of  sound  waves  possessing 
a  gradual  increase  of  pitch,  as  in  the  ascent  of  the  music  scale,  the 
tension  of  the  tympanic  membrane  is  gradually  increased  until  its 
maximum  tension  is  reached  at  the  upper  limit  of  the  range  of 
audibility.  By  this  change  in  tension  certain  tones  become  per- 
ceptible and  distinct,  while  others  become  indistinct  and  inaudible. 

Function  of  the  Tensor  Tympani  Muscle. — The  function  of 
this  muscle  is,  as  its  name  indicates,  to  increase  the  tension  of  the 


254  HUMAN   PHYSIOLOGY. 

membrane  in  accordance  with  the  pitch  of  the  sound  wave.  The 
tension  of  this  muscle  playing  over  the  processus  cochleariformis  and 
attached  at  also  a  right  angle  to  the  handle  of  the  malleus  will, 
when  the  muscle  contracts,  pull  the  handle  inward,  increase  the  con- 
vexity of  the  membrane,  and  at  the  same  time  increase  its  tension ; 
with  the  relaxation  of  this  muscle,  the  handle  of  the  malleus  passes 
outward  and  the  tension  is  diminished.  The  contractions  of  the 
tensor  muscle  are  reflex  in  character  and  excited  by  nerve  impulses 
reaching  it  through  the  small  petrosal  nerve  and  otic  ganglion.  The 
number  of  nerve  stimuli  passing  to  the  muscle  and  determining  the 
degree  of  contraction  will  depend  upon  the  pitch  of  the  sound  wave 
and  the  subsequent  excitation  of  the  auditory  nerve.  The  tensor 
tympani  muscle  may  be  regarded  as  an  accommodative  apparatus 
by  which  the  tympanic  membrane  is  so  adjusted  as  to  enable  it  to 
receive  vibrations  of  varying  degrees  of  pitch. 

Function  of  the  Ossicles.— The  function  of  the  chain  of  bones 
is  to  transmit  the  sound  wave  across  the  tympanic  cavity  to  the  in- 
ternal ear.  The  first  of  these  bones,  the  malleus,  being  attached  to 
the  tympanic  membrane,  will  take  up  the  vibrations  much  more  readily 
than  if  no  membrane  intervened.  Owing  to  the  character  of  the 
articulations,  when  the  handle  of  the  malleus  is  drawn  inward,  the 
position  of  the  bones  is  so  changed  that  they  form  practically  a  solid 
rod,  and  are  therefore  much  better  adapted  for  the  transmission  of 
molecular  vibrations  than  if  the  articulations  remained  loose.  As 
the  stapes  bone  is  somewhat  shorter  than  the  malleus,  its  vibrations 
are  slighter  than  those  of  the  tympanic  membrane,  and  by  this 
arrangement  the  amplitude  of  the  vibrations  is  diminished,  but  their 
force  increased. 

The  function  of  the  stapedius  muscle  is,  according  to  Henle, 
to  fix  the  stapes  bone  so  as  to  prevent  too  great  a  movement  from 
being  communicated  to  it  from  the  incus  and  transmitted  to  the  peri- 
lymph.  It  may  be  looked  upon,  therefore,  as  a  protective  muscle. 

The  function  of  the  Eustachian  tube  is  to  maintain  a  free  com- 
munication between  the  cavity  of  the  middle  ear  and  the  nasopharynx. 
The  pressure  of  air  within  and  without  the  ear  is  thus  equalized,  and 
the  vibrations  of  the  tympanic  membrane  are  permitted  to  attain 
their  maximum,  one  of  the  conditions  essential  for  the  reception  of 
sound  waves.  The  impairment  in  the  acuteness  of  hearing  which  is 


THE   SENSE  OF   HEARING.  255 

caused  by  an  unequal  pressure  of  the  air  in  the  middle  ear  can  be 
shoWn — 

1.  By  closing  the  mouth   and  nose   and   forcing   air   from   the  lungs 
through  the   Eustachian   tube   into   the   ear,   producing  an   increase 
in  pressure. 

2.  By  closing  the  nose  and  mouth,  and  making  efforts  at  deglutition, 
which  withdraws  the  air  from  the  ear  and  diminishes  its  pressure. 
In   both  instances  the   free   vibrations  of  the  tympanic   membrane 

are  interfered  with.  The  pharyngeal  orifice  of  the  Eustachian  tube 
is  opened  by  the  action  of  certain  of  the  muscles  of  deglutition — 
viz.,  the  levator  palati,  the  tensor  palati,  and  the  palato-pharyngei 
muscles. 

The  internal  ear,  or  labyrinth,  is  located  in  the  petrous  portion 
of  the  temporal  bone,  and  consists  of  an  osseous  and  a  membranous 
portion. 


The  osseous  labyrinth  is  divisible  into  three  parts — viz.,  the  vesti- 
bule, the  semicircular  canals,  and  the  cochlea. 

The  vestibule  is  a  small,  triangular  cavity,  which  communicates 
with  the  middle  ear  by  the  foramen  ovule  ;  in  the  natural  condition 
it  is  closed  by  the  base  of  the  stapes  bone.  The  filaments  of  the 
auditory  nerve  enter  the  vestibule  through  small  foramina  in  the 
inner  wall,  at  the  fovea  hemispherica. 

The  semicircular  canals  are  three  in  number,  the  superior  vertical, 
the  inferior  vertical,  and  the  horizontal,  each  of  which  opens  into 
the  cavity  of  the  vestibule  by  two  openings,  with  the  exception  of 
the  two  vertical,  which  at  one  extremity  open  by  a  common  orifice. 

The  cochlea  forms  the  anterior  part  of  the  internal  ear.  It  is  a 
gradually  tapering  canal,  about  iT/2  inches  in  length,  which  winds 
spirally  around  a  central  axis,  the  modiolus,  two  and  one  half  times. 
•  The  interior  of  the  cochlea  is  partly  divided  into  two  passages  by 
a  thin  plate  of  bone,  the  lamina  osseous  spiralis,  which  projects 
from  the  central  axis  two  thirds  of  the  way  across  the  canal.  These 
passages  are  termed  the  scala  vestibuli  and  the  scala  tympani,  from 
their  communication  with  the  vestibule  and  tympanum  The  scala 
tympani  communicates  with  the  middle  ear  through  the  foramen  ro- 
tundum,  which,  in  the  natural  condition,  is  closed  by  the  second 
membrana  tympani ;  superiorly  they  are  united  by  an  opening,  the 
helicotrema. 

The    whole    interior    of    the    labyrinth,    the    vestibule,    the    semi- 


256  HUMAN   PHYSIOLOGY. 

circular  canals,  and  the  scala  of  the  cochlea,  contains  a  clear,  limpid 
fluid,  the  perilymph  secreted  by  the  periosteum  lining  the  osseous 
walls. 

The  membranous  labyrinth  corresponds  to  the  osseous  labyrinth 
with  respect  to  form,  though  it  is  somewhat  smaller  in  size. 

The  vestibular  portion  consists  of  two  small  sacs,  the  utricle  and 
the  saccule. 

The  semicircular  canals  communicate  with  the  utricle  in  the  same 
manner  as  the  bony  canals  communicate  with  the  vestibule.  The 
saccule  communicates  with  the  membranous  cochlea  by  the  canalis 
reuniens.  In  the  interior  of  the  utricle  and  saccule,  at  the  entrance 
of  the  auditory  nerve,  are  small  masses  of  carbonate  of  lime  crys- 
tals, constituting  the  otoliths.  Their  function  is  unknown. 

The  membranous  cochlea  is  a  closed  tube,  commencing  by  a  .blind 

extremity   at   the   first   turn   of   the   cochlea,    and   terminating   at   its 

.  apex  by  a  blind  extremity  also.     It  is  situated  between  the  edge  of  the 

osseous  lamina  spiralis  and  the  outer  wall  of  the  bony  cochlea,  and 

follows  it  in  its  turns  around  the  modiolus. 

A  transverse  section  of  the  cochlea  shows  that  it  is  divided  into 
two  portions  by  the  osseous  lamina  and  the  basilar  membrane : 

1.  The   scala   vestibuli,   bounded    by   the   periosteum    and    membrane 
of  Reissner. 

2.  The  scala  tympania,  occupying  the  inferior  portion,  and  bounded 
above   by   the   septum,    composed    of   the   osseous   lamina    and   the 
membrana  basilaris. 

The  true  membranous  canal  is  situated  between  the  membrane  of 
Reissner  and  the  basilar  membrane.  It  is  triangular  in  shape,  but 
is  partly  divided  into  a  triangular  portion  and  a  quadrilateral  portion 
by  the  tectorial  membrane. 

The  organ  of  Corti  is  situated  in  the  quadrilateral  portion  of  the. 
canal,  and  consists  of  pillars  of  rods  of  the  consistence  of  cartilage. 
They  are  arranged  in  two  rows — the  one  internal,  the  other  external ; 
these  rods  rest  upon  the  basilar  membrane ;  their  bases  are  separated 
from  one  another,  but  their  upper  extremities  are  united,  forming  an 
arcade.  In  the  internal  row  it  is  estimated  there  are  about  3,500 
and  in  the  external  row  about  5,200  of  these  rods. 

On  the  inner  side  of  the  internal  row  is  a  single  layer  of  elongated 
hair-cells ;  on  the  outer  surface  of  the  external  row  are  three  such 
layers  of  hair-cells.  Nothing  definite  is  known  as  to  their  function. 


THE   SENSE  OF   HEARING.  257 

The  endolymph  occupies  the  interior  of  the  utricle,  saccule,  and 
membranous  canals,  and  bathes  the  structures  in  the  interior  of  the 
membranous  cochlea  throughout  its  entire  extent. 

The  auditory  nerve  at  the  bottom  of  the  internal  auditory  meatus 
divides  into — 

1.  A  vestibular  branch,  which  is  distributed  to  the  utricle  and  to  the 
semicircular  canals. 

2.  A  cochlear  branch,  which  passes  into  the  central  axis  at  its  base 
and  ascends  to  its  apex ;  as  it  ascends,  fibers  are  given  off,  which 
pass   between   the   plates   of   the   osseous   lamina,   to   be   ultimately 
connected  with  the  organ  of  Corti. 

The  function  of  the  semicircular  canals  appears  to  be  to  assist 
in  maintaining  the  equilibrium  of  the  body ;  destruction  of  the 
vertical  canal  is  followed  by  an  oscillation  of  the  head  upward  and 
downward;  destruction  of  the  horizontal  canal  is  followed  by  oscil- 
lations from  left  to  right.  When  the  canals  are  injured  on  both 
sides,  the  animal  loses  the  power  of  maintaining  equilibrium  upon 
making  muscular  movements. 

Function  of  the  Cochlea. — It  is  regarded  as  possessing  the  power 
of  appreciating  the  quality  of  pitch  and  the  shades  of  different 
musical  tones.  The  elements  of  the  organ  of  Corti  are  analogous,  in 
some  respects,  to  a  musical  instrument,  and  are  supposed,  by  Helm- 
holtz,  to  be  tuned  so  as  to  vibrate  in  unison  with  the  different  tones 
conveyed  to  the  internal  ear. 

Summary. — The  waves  of  sound  are  gathered  together  by  the 
pinna  and  external  auditory  meatus,  and  conveyed  to  the  membrana 
tympani.  This  membrane,  made  tense  or  lax  by  the  action  of  the 
tensor  tympani  and  laxator  tympani  muscles,  is  enabled  to  receive 
sound  waves  of  either  high  or  low  pitch.  The  vibrations  are  con- 
ducted across  the  middle  ear  by  a  chain  of  bones  to  the  foramen 
ovale,  and  by  the  column  of  air  of  the  tympanum  to  the  foramen 
rotundum,  which  is  closed  by  the  second  membrana  tympani,  the 
pressure  of  the  air  in  the  tympanum  being  regulated  by  the  Eu- 
stachian  tube. 

The  internal  ear  finally  receives  the  vibrations,  which  excite  vi- 
brations successively  in  the  perilymph,  the  walls  of  the  membranous 
labyrinth,  the  endolymph,  and,  lastly,  the  terminal  filaments  of  the 
auditory  nerve,  by  which  they  are  conveyed  to  the  brain. 

17 


258  HUMAN    PHYSIOLOGY. 

VOICE   AND    SPEECH. 

The  larynx  is  the  organ  of  voice.  Speech  is  a  modification  of 
voice,  and  is  produced  by  the  teeth  and  the  muscles  of  the  lips  and 
tongue,  coordinated  in  their  action  by  stimuli  derived  from  the 
cerebrum. 

The  structures  entering  into  the  formation  of  the  larynx  are 
mainly  the  thyroid,  cricoid,  and  arytenoid  cartilages ;  they  are  so 
situated  and  united  by  means  of  ligaments  and  muscles  as  to  form  a 
firm  cartilaginous  box.  The  larynx  is  covered  externally  by  fibrous 
tissue,  and  lined  internally  with  mucous  membrane. 

The  vocal  cords  are  four  ligamentous  bands,  running  anteropos- 
teriorly  across  the  upper  portion  of  the  larynx,  and  are  divided 
into  the  two  superior  or  false  vocal  cords,  and  the  two  inferior  or  true 
vocal  cords ;  they  are  attached  anteriorly  to  the  receding  angle  of 
the  thyroid  cartilages,  and  posteriorly  to  the  anterior  part  of  the 
base  of  the  arytenoid  cartilages.  The  space  between  the  true  vocal 
cords  is  the  rima  glottidis. 

The  muscles  which  have  a  direct  action  upon  the  movements  of 
the  vocal  cords  are  nine  in  number,  and  take  their  names  from  their 
points  of  origin  and  insertion — viz.,  the  two  crico-thyroid,  two 
thyro-arytenoid,  two  posterior  crico-arytenoid,  two  lateral  crico-ary- 
tenoid,  and  one  arytenoid  muscles. 

The  crico-thyroid  muscles,  by  their  contraction,  render  the  vocal 
cords  more  tense  by  drawing  down  the  anterior  portion  of  the 
thyroid  cartilage  and  approximating  it  to  the  cricoid,  and  at  the 
same  time  tilting  the  posterior  portion  of  the  cricoid  and  arytenoid 
cartilages  backward. 

The  thyro-arytenoid,  by  their  contraction,  relax  the  vocal  cords 
by  drawing  the  arytenoid  cartilage  forward  and  the  thyroid  back- 
ward. 

The  posterior  crico-arytenoid  muscles,  by  their  contraction,  rotate 
the  arytenoid  cartilages  outward  and  thus  separate  the  vocal  cords 
and  enlarge  the  aperture  of  the  glottis.  They  principally  aid  the 
respiratory  movements  during  inspiration. 

The  lateral  crico-arytenoid  muscles  are  antagonistic  to  the  former, 
and  by  their  contraction  rotate  the  arytenoid  cartilages  so  as  to  ap- 
proximate the  vocal  cords  and  constrict  the  glottis, 


VOICE  AND   SPEECH.  259 

The  arytenoid  muscle  assists  in  the  closure  of  the  aperture  of  the 
glottis. 

The  inferior  laryngeal  nerve  animates  all  the  muscles  of  the  larynx, 
with  the  exception  of  the  crico-thyroid. 

Movements  of  the  Vocal  Cords. — During  respiration  the  move- 
ment of  the  vocal  cords  differ  from  those  occurring  during  the  pro- 
duction of  voice. 

At  each  inspiration  the  true  vocal  cords  are  widely  separated,  and 
the  aperture  of  the  glottis  is  enlarged  by  the  action  of  the  crico- 
arytenoid  muscles,  which  rotate  outward  the  anterior  angle  of 
the  base  of  the  arytenoid  cartilages  ;  at  each  expiration  the  larynx 
becomes  passive ;  the  elasticity  of  the  vocal  cords  returns  them  to 
their  original  position,  and  the  air  is  forced  out  by  the  elasticity  of 
the  lungs  and  the  walls  of  the  thorax. 

Phonation. — As  soon  as  phonation  is  about  to  be  accomplished, 
a  marked  change  in  the  glottis  is  noticed  with  the  aid  of  the 
laryngoscope.  The  true  vocal  cords  suddenly  become  approximated 
and  are  made  parallel,  giving  to  the  glottis  the  appearance  of  a  nar- 
row slit,  the  edges  of  which  are  capable  of  vibrating  accurately  and 
rapidly  ;  at  the  same  time  their  tension  is  much  increased. 

With  the  vocal  cords  thus  prepared,  the  expiratory  muscles  force 
the  column  of  air  into  the  lungs  and  trachea  through  the  glottis, 
throwing  the  edges  of  the  cords  into  vibration. 

The  pitch  of  sounds  depends  upon  the  extent  to  which  the  vocal 
cords  are  made  tense  and  the  length  of  the  aperture  through  which  the 
air  passes.  In  the  production  of  sounds  of  a  high  pitch,  the  tension 
of  the  vocal  cords  becomes  very  marked  and  the  glottis  diminished 
in  length.  When  sounds  having  a  low  pitch  are  emitted  from  the 
larynx,  the  vocal  cords  are  less  tense  and  their  vibrations  are  large 
and  loose. 

The  quality  of  voice  depends  upon  the  length,  size,  and  thickness 
of  the  cords,  and  upon  the  size,  form,  and  construction  of  the 
trachea,  the  larynx,  and  the  resonant  cavities  of  the  pharynx,  nose, 
and  mouth. 

The  compass  of  the  voice  comprehends  from  two.  to  three  octaves. 
The  range  is  different  in  the  two  sexes,  the  lowest  note  of  the 
male  being  about  one  octave  lower  than  the  lowest  note  of  the 
female  ;  while  the  highest  note  of  the  male  is  an  octave  less  than  the 
highest  note  of  the  female. 


260  HUMAN    PHYSIOLOGY. 

The  varieties  of  voice — e.  g.}  bass,  baritone,  tenor,  contralto,  mezzo- 
soprano,  and  soprana — are  due  to  the  length  of  the  vocal  cords, 
being  longer  when  the  voice  has  a  low  pitch,  and  shorter  when  it  has 
a  high  pitch. 

Speech  is  the  faculty  of  expressing  ideas  by  means  of  combina- 
tions of  sounds,  in  obedience  to  the  dictates  of  the  cerebrum. 

Articulate  sounds  may  be  divided  into  vowels  and  consonants.  The 
vowel  sounds,  a,  e,  i,  o,  u,  are  produced  in  the  larynx  by  the  vocal 
cords.  The  consonant  sounds  are  produced  in  the  air-passages  above 
the  larynx  by  an  interruption  of  the  current  of  air  by  the  lips,  tongue, 
and  teeth ;  the  consonants  may  be  divided  into  : 

1.  Mutes,  b,  d,  k,  p,  t,  c,  g. 

2.  Dentals,  d,  j,  s,  t,  z. 

3.  Nasals,  m,  n,  ng. 

4.  Labials,  b,  p,  f,  v,  m. 

5.  Gutturals,  k,  g,  c,  and  g  hard. 

6.  Liquids,   /_,  m,  n}  r. 


EMBRYOLOGY. 

Reproduction  is  the  function  by  which  the  species  is  preserved  ; 
it  is  accomplished  by  the  organs  of  generation  in  the  two  sexes. 
Embryology  is  the  science  which  investigates  the  successive  stages 
in  the  development  of  the  embryo. 


GENERATIVE    ORGANS    OF    THE    FEMALE. 

The  generative  organs  of  the  female  consist  of  the  ovaries, 
Fallopian  tubes,  uterus,  and  vagina. 

The  ovaries  are  two  small,  ovoid,  flattened  bodies,  measuring  i% 
inches  in  length  and  %  of  an  inch  in  width  ;  they  are  situated  in  the 
cavity  of  the  pelvis,  and  are  imbedded  in  the  posterior  layer  of  the 
broad  ligament ;  attached  to  the  uterus  by  a  round  ligament,  and  to 
the  extremities  of  the  Fallopian  tubes  by  the  fimbrise.  The  ovary 
Consists  of  an  external  membrane  of  fibrous  tissue,  the  cortical 
portion,  in  which  are  embedded  the  Graafian  vesicles,  and  an  internal 
portion,  the  stroma,  containing  blood-vessels. 

The  Graafian  vesicles  are  exceedingly  numerous,  but  are  situated 
only  in  the  cortical  portion.  Although  the  ovary  contains  the 
vesicles  from  the  period  of  birth,  it  is  only  at  puberty  that  they  attain 
their  full  development.  From  this  time  onward  to  the  catamenial 
period  there  is  a  constant  growth  and  maturation  of  the  Graafian 
vesicles.  They  consist  of  an  external  investment,  composed  of  fibrous 
tissues  and  blood-vessels,  in  the  interior  of  which  is  a  layer  of  cells 
forming  the  membrana  granulosa ;  at  its  lower  portion  there  is  an 
accumulation  of  cells,  the  proligerous  disc,  in  which  the  ovum  is 
contained.  The  cavity  of  the  vesicle  contains  a  slightly  yellowish 
alkaline,  albuminous  fluid. 

The  ovum  is  a  globular  body,  measuring  about  Ti?  of  an  inch 
in  diameter ;  it  consists  of  an  external  investing  membrane,  the 
vitelline  membrane;  a  central  granular  substance,  the  vitellus,  or 
yolk;  a  nucleus,  the  germinal  vesicle,  in  the  interior  of  which  is 
imbedded  the  nucleolus,  or  germinal  spot. 

261 


262  HUMAN   PHYSIOLOGY. 

The  Fallopian  tubes  are  about  four  inches  in  length,  and  extend 
outward  from  the  upper  angles  of  the  uterus,  between  the  folds  of 
the  broad  ligaments,  and  terminate  in  a  fringed  extremity  which  is 
attached  by  one  of  the  fringes  to  the  ovary.  They  consist  of  three 
coats : 

1.  The  external,  or  peritoneal. 

2.  Middle,   or  muscular,   the  fibers   of  which   are  arranged  in   a   cir- 
cular or  longitudinal  direction. 

3.  Internal,   or  mucous,   covered  with   ciliated  epithelial   cells,   which 
are  always  waving  from  the  ovary  toward  the  uterus. 

The  Uterus  is  pyriform  in  shape,  and  may  be  divided  into  a  body 
and  neck ;  it  measures  about  three  inches  in  length  and  two  inches 
in  breadth  in  the  unimpregnated  state.  At  the  lower  extremity  of 
the  neck  is  the  os  externum  ;  at  the  junction  of  the  neck  with  the 
body  is  a  constriction,  the  os  internuni.  The  cavity  of  the  uterus 
is  triangular  in  shape,  the  walls  of  the  triangle  being  almost  in 
contact. 

The  walls  of  the  uterus  are  made  up  of  several  layers  of  non- 
striated  muscle-fibers,  covered  externally  by  peritoneum,  and  lined 
internally  by  mucous  membrane,  containing  numerous  tubular  glands, 
and  covered  by  ciliated  epithelial  cells. 

The  vagina  is  a  membranous  canal,  from  five  to  six  inches  in 
length,  situated  between  the  rectum  and  bladder.  It  extends  obliquely 
upward  from  the  surface,  almost  to  the  brim  of  the  pelvis,  and  em- 
braces at  its  upper  extremity  the  neck  of  the  uterus. 

Discharge  of  the  Ovum. — As  the  Graafian  vesicle  matures  it 
increases  in  size,  from  an  augmentation  of  its  liquid  contents,  and 
approaches  the  surface  of  the  ovary,  where  it  forms  a  projection, 
measuring  from  J4  to  J4  of  an  inch.  The  maturation  of  the  vesicle 
occurs  periodically,  about  every  twenty-eight  days,  and  is  attended 
by  the  phenomena  of  menstruation.  During  this  period  of  active 
congestion  of  the  reproductive  organs  the  Graafian  vesicle  ruptures, 
the  ovum  and  liquid  contents  escape,  and  are  caught  by  the  fimbriated 
extremity  of  the  Fallopian  tube,  which  has  adapted  itself  to  the  pos- 
terior surface  of  the  ovary.  The  passage  of  the  ovum  through  the 
Fallopian  tube  into  the  uterus  occupies  from  ten  to  fourteen  days, 
and  is  accomplished  by  muscular  contraction  and  by  the  action  of 
the  ciliated  epithelium. 


EMBRYOLOGY. 


263 


Menstruation  is  a  periodic  discharge  of  blood  from  the  mucous 
membrane  of  the  uterus,  due  to  a  fatty  degeneration  of  the  small 
blood-vessels.  Under  the  pressure  of  an  increased  amount  of  blood 
in  the  reproductive  organs,  attending  the  process  of  ovulation,  the 
blood-vessels  rupture,  and  a  hemorrhage  takes  place  into  the  uterine 
cavity ;  thence  it  passes  into  the  vagina.  Menstruation  lasts  from 
five  to  six  days,  and  the  amount  of  blood  discharged  averages  about 
five  ounces. 

Corpus  Luteum. — For  some  time  previous  to  the  rupture  of  a 
Graafian  vesicle  it  increases  in  size  and  becomes  vascular ;  its  walls 
become  thickened  from  the  deposition  of  a  reddish-yellow,  glutinous 
substance,  a  product  of  cell  growth  from  the  proper  coat  of  the 
follicle  and  the  membrana  granulosa.  After  the  ovum  escapes  there 
is  usually  a  small  effusion  of  blood  into  the  cavity  of  the  follicle, 
which  soon  coagulates,  loses  its  coloring-matter,  and  acquires  the 
characteristics  of  fibrin,  but  it  takes  no  part  in  the  formation  of  the 
corpus  luteum.  The  walls  of  the  follicle  become  convoluted  and 
vascular,  and  undergo  hypertrophy,  until  they  occupy  the  whole  of 
the  follicular  cavity.  At  its  period  of  fullest  development  the  corpus 
luteum  measures  54  of  an  inch  in  length  and  ^  of  an  inch  in  depth. 
In  a  few  weeks  the  mass  loses  its  red  color  and  becomes  yellow,  con- 
stituting the  corpus  luteum,  or  yellow  body.  It  then  begins  to  retract 
and  becomes  pale ;  and  at  the  end  of  two  months  nothing  remains  but 
a  small  cicatrix  upon  the  surface  of  the  ovary.  Such  are  the 
changes  in  the  follicle  if  the  ovum  has  not  been  impregnated. 

The  corpus  luteum,  after  impregnation  has  taken  place,  undergoes 
a  much  slower  development,  becomes  larger,  and  continues  during 
the  entire  period  of  gestation.  The  difference  between  the  corpus 
luteum  of  the  unimpregnated  and  pregnant  condition  is  expressed 
in  the  following  table  by  Dalton : 

Corpus  Luteum  of  Menstruation.     Corpus  Luteum  of  Pregnancy. 


At   the   end   of 
three  weeks. 
One  month. 


Two  months. 


Three  quarters  of  an  inch  in  diameter ;  central  clot 
reddish  ;  convoluted  wall  pale. 


Smaller ;  convoluted 
wall  bright  yellow ; 
clot  still  reddish. 

Reduced  to  the  con- 
dition -of  an  insignifi- 
cant cicatrix. 


Larger ;  convoluted  wall 
bright  yello w  ;  clot  still  reddish. 

Seven  eighths  of  an  inch  in 
diameter ;  convoluted  wall 
bright  yellow;  clot  perfectly 
decolorized. 


264 


HUMAN   PHYSIOLOGY. 


Four  months. 


Six  months. 


Nine  months. 


Absent  or  unnotice- 
able. 


Absent. 


Absent. 


Seven  eighths  of  an  inch  in 
diameter  ;  clot  pale  and  fibrin- 
ous  ;  convoluted  wall  dull  yel- 
low. 

Still  as  large  as  at  the  end  of 
second  month  ;  clot  fibrinous  ; 
convoluted  wall  paler. 

Half  an  inch  in  diameter ; 
central  clot  converted  into  a 
radiating  cicatrix ;  external 
wall  tolerably  thick  and  con- 
voluted, but  without  any  bright 
yellow  color. 


GENERATIVE    ORGANS    OF    THE   MALE. 

The  generative  organs  of  the  male  consist  of  the  testicles,  vasa 
deferentia,  vesiculae  seminales,  and  penis. 

The  testicles,  the  essential  organs  of  reproduction  in  the  male, 
are  two  oblong  glands,  about  il/2  inches  in  length,  compressed  from 
side  to  side,  and  situated  in  the  cavity  of  the  scrotum. 

The  proper  coat  of  the  testicle,  the  tunica  albuginea,  is  a  white, 
fibrous  structure,  about  -fa  of  an  inch  in  thickness  ;  after  enveloping 
the  testicle,  it  is  reflected  into  its  interior  at  the  posterior  border, 
and  forms  a  vertical  process,  the  mediastinum  testis,  from  which 
septa  are  given  off,  dividing  the  testicle  into  lobules. 

The  substance  of  the  testicle  is  made  up  of  the  seminiferous  tubules, 
which  exist  to  the  number  of  840  ;  they  are  exceedingly  convoluted, 
and  when  unravelled  are  about  thirty  inches  in  length.  As  they 
pass  toward  the  apices  of  the  lobules,  they  become  less  convoluted, 
and  terminate  in  from  twenty  to  thirty  straight  ducts,  the  vasa  recta, 
which  pass  upward  through  the  mediastinum  and  constitute  the 
rete  testis.  At  the  upper  part  of  the  mediastinum  the  lobules  unite 
to  form  from  nine  to  thirty  small  ducts,  the  vasa  efferentia,  which 
become  convoluted  and  form  the  globus  major  of  the  epididymis ;  the 
continuation  of  the  tubes  downward  behind  the  testicle  and  a 
second  convolution  constitutes  the  body  and  globus  minor. 

The  seminal  tubule  consists  of  a  basement  membrane  lined  by 
granular  nucleated  epithelium. 


EMBRYOLOGY.  265 

The  vas  deferens,  the  excretory  duct  of  the  testicle,  is  about  two 
feet  in  length,  and  may  be  traced  upward  from  the  epididymis  to 
the  under  surface  of  the  base  of  the  bladder,  where  it  unites  with 
the  duct  of  the  vesicula  seminalis  to  form  the  ejaculatory  duct. 

The  vesiculae  seminales  are  two  lobulated,  pyriform  bodies  about 
two  inches  in  length,  situated  on  the  inner  surface  of  the  bladder. 

They  have  an  external  fibrous  coat,  a  middle  muscular  coat,  and 
an  internal  mucous  coat,  covered  by  epithelium,  which  secretes  a 
mucous  fluid.  The  vesiculae  seminales  serve  as  reservoirs,  in  which 
the  seminal  fluid  is  temporarily  stored  up. 

The  ejaculatory  duct,  about  1/4  of  an  inch  in  length,  opens  into 
the  urethra,  and  is  formed  by  the  union  of  the  vasa  deferentia  and 
the  ducts  of  the  vesiculae  seminales. 

The  prostate  gland  surrounds  the  posterior  extremity  of  the 
urethra,  and  opens  into  it  by  from  twenty  to  thirty  openings,  the 
orifices  of  the  prostatic  tubules.  The  gland  secretes  a  fluid  which 
forms  part  of  the  semen  and  assists  in  maintaining  the  vitality  of 
the  spermatozoa. 

Semen  is  a  complex  fluid,  made  up  of  the  secretions  from  the 
testicles,  the  vesiculae  seminales,  the  prostatic  and  urethral  glands. 
It  is  grayish-white  in  color,  mucilaginous  in  consistence,  of  a  char- 
acteristic odor,  and  somewhat  heavier  than  water.  From  half  a 
dram  to  a  dram  is  -ejaculated  at  each  orgasm. 

The  spermatozoa  are  peculiar  anatomic  elements,  developed  within 
the  seminal  tubules,  and  possess  the  power  of  spontaneous  move- 
ment. The  spermatozoa  consist  of  a  conoid  head  and  a  long,  fila- 
mentous tail,  which  is  in  continuous  and  active  motion ;  so  long 
as  they  remain  in  the  vas  deferens  they  are  quiescent,  but  when 
free  to  move  in  the  fluid  of  the  vesiculae  seminales,  they  become  very 
active. 

Origin. — The  spermatozoa  appear  at  the  age  of  puberty,  and  are 
then  constantly  formed  until  an  advanced  age.  They  are  developed 
from  the  nuclei  of  large,  round  cells  contained  in  the  anterior  of 
the  seminal  tubules,  as  many  as  fifteen  to  twenty  developing  in  a 
single  cell. 

When  the  spermatozoa  are  introduced  into  the  vagina,  they  pass 
readily  into  the  uterus  and  'through  the  Fallopian  tubes  toward  the 
ovaries,  where  they  remain  and  retain  their  vitality  for  a  period 
of  from  eight  to  ten  days. 


266  HUMAN   PHYSIOLOGY. 

Fecundation  is  the  union  of  the  spermatozoa  with  the  ovum  during 
its  passage  toward  the  uterus,  and  usually  takes  place  in  the  Fal- 
lopian tube,  just  outside  the  womb.  After  floating  around  the  ovum 
in  an  active  manner,  they  penetrate  the  vitelline  membrane,  pass  into 
the  interior  of  the  vitellus,  where  they  lose  their  vitality,  and,  along 
with  the  germinal  vesicle,  entirely  disappear. 

DEVELOPMENT    OF   ACCESSORY    STRUCTURES. 

Segmentation  of  the  Vitellus. — After  the  disappearance  of  the 
spermatozoa  and  the  germinal  vesicle  there  remains  a  transparent, 
granular,  albuminous  substance,  in  the  center  of  which  a  new  nucleus 
soon  appears ;  this  constitutes  the  parent  cells,  and  is  the  first  stage 
in  the  development  of  the  new  being. 

Following  this,  the  vitellus  undergoes  segmentation ;  a  constric- 
tion appears  on  the  opposite  side  of  the  vitellus,  which  gradually 
deepens,  until  the  yolk  is  divided  into  two  segments,  each  of  which 
has  a  distinct  nucleus  and  nucleolus ;  these  two  segments  undergo 
a  further  division  into  four,  the  four  into  eight,  the  eight  into 
others,  and  so  on,  until  the  entire  vitellus  is  divided  into  a  great 
number  of  cells,  each  of  which  contains  a  nucleus  and  a  nucleolus. 

The  peripheral  cells  of  this  "  mulberry  mass  "  then  arrange  them- 
selves so  as  to  form  a  membrane,  and,  as  they  are  subjected  to  mutual 
pressure,  assume  a  polyhedral  shape,  which  gives  to  the  membrane 
a  mosaic  appearance.  The  central  part  of  the  vitellus  becomes 
filled  with  a  clear  fluid.  A  second  membrane  shortly  appears  within 
the  first,  and  the  two  together  constitute  the  external  and  internal 
blastodermic  membranes. 

Blastodermic  Membranes. — The  embryo,  at  this  period,  consists 
of  three  layers — viz.,  the  external  and  the  internal  blastodermic 
membranes  and  a  middle  membrane  formed  by  a  genesis  of  cells 
from  their  internal  surfaces.  These  layers  are  known  as  the  epi- 
blast,  mesoblast,  and  hypoblast. 

The  epiblast  gives  rise  to  the  central  nervous  system,  the  epidermis 
of  the  skin  and  its  appendages,  and  the  primitive  kidneys. 

The  mesoblast  gives  rise  to  the  dermis,  muscles,  bones,  nerves, 
blood-vessels,  sympathetic  nervous  system,  connective  tissue,  the 
urinary  and  reproductive  apparatus,  and  the  walls  of  the  alimentary 
canal. 


EMBRYOLOGY.  267 

The  hypoblast  gives  rise  to  the  epithelial  lining  of  the  alimentary 
canal  and  its  glandular  appendages,  the  liver  and  pancreas,  and  the 
epithelium  of  the  respiratory  tract. 

Germinal  Area. — At  about  this  period  there  is  an  accumulation 
of  cells  at  a  certain  spot  upon  the  surface  of  the  blastodermic 
membranes,  which  marks  the  position  of  the  future  embryo.  This 
spot,  at  first  circular,  soon  becomes  elongated,  and  forms  the  primi- 
tive trace,  around  which  is  a  clear  space,  the  area  pellucida,  which 
is  itself  surrounded  by  a  darker  region,  the  area  opaca. 

The  primitive  trace  soon  disappears,  and  the  area  pellucida  be- 
comes guitar-shaped ;  a  new  groove,  the  medullary  groove,  is  now 
formed,  which  develops  from  before  backward,  and  becomes  the 
neural  canal. 

Dorsal  Laminae. — As  development  advances,  the  true  medullary 
groove  deepens,  and  there  arise  two  longitudinal  elevations  of  the 
epiblast, — the  dorsal  lamina,  one  on  either  side  of  the  groove, — 
which  grow  up,  arch  over,  and  unite  so  as  to  form  a  closed  tube, 
the  primitive  central  nervous  system. 

The  chorda  dorsalis  is  a  cylindric  rod  running  almost  throughout 
the  entire  length  of  the  embryo.  It  is  formed  by  an  aggregation  of 
mesoblastic  cells,  and  is  situated  immediately  beneath  the  medul- 
lary groove. 

Primitive  Vertebrae.— On  either  side  of  the  neural  canal  the  cells 
of  the  mesoblast  undergo  a  longitudinal  thickening,  which  develops 
and  extends  around  the  neural  canal  and  the  chorda  dorsalis,  and 
forms  the  arches  and  bodies  of  the  vertebrae.  They  become  divided 
transversely  into  four-sided  segments. 

The  mesoblast  now  separates  into  two  layers  :  the  external,  joining 
with  the  epiblast,  forms  the  somatopleura ;  the  internal,  joining 
with  the  hypoblast,  forms  the  splanchnopleura ;  the  space  between 
them  constitutes  the  pi  euro -peritoneal  cavity. 

Visceral  Laminae. — The  walls  of  the  pleuro-peritoneal  cavity  are 
formed  by  a  downward  prolongation  of  the  somatopleura  (the 
visceral  lamina),  which,  as  they  extend  around  in  front,  pinch  off  a 
portion  of  the  yolk-sac  (formed  by  the  splanchnopleura),  which  be- 
comes the  primitive  alimentary  canal ;  the  lower  portion,  remaining 
outside  of  the  body  cavity,  forms  the  umbilical  vesicle,  which  after  a 
time  disappears. 


268  HUMAN   PHYSIOLOGY. 

Formation  of  Fetal  Membranes. — The  amnion  appears  shortly 
after  the  embryo  begins  to  develop,  and  is  formed  by  folds  of  the 
epiblast  and  external  layer  of  the  mesoblast,  rising  up  in  front  and 
behind  and  on  each  side ;  these  amniotic  folds  gradually  extend 
over  the  back  of  the  embryo  to  a  certain  point,  where  they  coalesce 
and  inclose  a  cavity — the  amniotic  cavity.  The  membranous  parti- 
tion between  the  folds  disappears,  and  the  outer  layer  recedes  and 
becomes  blended  with  the  vitelline  membrane,  constituting  the  chorion 
— the  external  covering  of  the  embryo. 

The  Allantois. — As  the  amnion  develops,  there  grows  out  from 
the  posterior  portion  of  the  alimentary  canal  a  pouch,  or  diverticulum 
(the  allantois},  which  carries  blood-vessels  derived  from  the  in- 
testinal circulation.  As  it  gradually  enlarges  it  becomes  more  vas- 
cular, and  inserts  itself  between  the  two  layers  of  the  amnion,  com- 
ing into  intimate  contact  with  the  external  layer.  Finally,  from 
increased  growth,  it  completely  surrounds  the  embryo,  and  its  edges 
become  fused  together. 

In  the  bird  the  allantois  is  a  respiratory  organ,  absorbing  oxygen 
and  exhaling  carbonic  acid;  it  also  absorbs  nutritive  matter  from  the 
interior  of  the  egg. 

Amniotic  Fluid. — The  amnion,  when  first  formed,  is  in  close 
contact  with  the  surface  of  the  ovum ;  but  it  soon  enlarges,  and  be- 
comes filled  with  a  clear,  transparent  fluid,  containing  albumin, 
glucose,  fatty  matters,  urea,  and  inorganic  salts.  It  increases  in 
amount  up  to  the  latter  period  of  gestation,  when  it  amounts  to 
about  two  pints.  In  the  space  between  the  amnion  and  allantois 
is  a  gelatinous  material,  which  is  encroached  upon  and  finally  dis- 
appears as  the  amnion  and  allantois  come  in  contact,  at  about  the 
fifth  month. 

The  chorion,  the  external  investment  of  the  embryo,  is  formed  by 
a  fusion  of  the  original  vitelline  membrane,  the  external  layer  of 
the  amnion,  and  the  allantois.  The  external  surface  now  becomes 
covered  with  villous  processes,  which  increase  in  number  and  size 
by  the  continual  budding  and  growth  of  club-shaped  processes  from 
the  main  stem,  and  give  to  the  chorion  a  shaggy  appearance.  They 
consist  of  a  homogeneous  granular  matter,  and  are  penetrated  by 
branches  of  the  blood-vessels  derived  from  the  aorta. 

The   presence   of  villous   processes   in   the  uterine   cavity   is  proof 


EMBRYOLOGY.  269 

positive  of  the  previous  existence  of  a  fetus.  They  are  characteristic 
of  the  chorion,  and  are  found  under  no  other  circumstances. 

At  about  the  end  of  the  second  month  the  villosities  begin  to 
atrophy  and  disappear  from  the  surface  of  the  chorion,  with  the  ex- 
ception of  those  situated  at  the  points  of  entrance  of  the  fetal 
blood-vessels,  which  occupy  about  one  third  of  its  surface,  where 
they  continue  to  grow  longer,  become  more  vascular,  and  ultimately 
assist  in  the  formation  of  the  placenta ;  the  remaining  two  thirds 
of  the  surface  loses  its  villi  and  blood-vessels  and  becomes  a 
simple  membrane. 

The  umbilical  cord  connects  the  fetus  with  that  portion  of  the 
chorion  which  forms  the  fetal  side  of  the  placenta.  It  is  a  process 
of  the  allantois,  and  contains  two  arteries  and  a  vein,  which  have  a 
more  or  less  spiral  direction.  It  appears  at  the  end  of  the  first 
month,  and  gradually  increases  in  length  until,  at  the  end  of  gesta- 
tion, it  measures  about  twenty  inches.  The  cord  is  also  surrounded 
by  a  process  of  the  amnion. 

Development  of  the  Decidual  Membrane. — The  interior  of  the 
uterus  is  lined  by  a  thin,  delicate  mucous  membrane,  in  which  are 
embedded  immense  numbers  of  tubules,  terminating  in  blind  ex- 
tremities— the  uterine  tubules.  At  each  period  of  menstruation  the 
mucous  membrane  becomes  thickened  and  vascular,  which  condition, 
however,  disappears  after  the  usual  menstrual  discharge.  When  the 
ovum  becomes  fecundated,  the  mucous  membrane  takes  on  an  in- 
creased growth,  becomes  more  hypertrophied  and  vascular,  sends  up 
little  processes  or  elevations  from  its,  surf  ace,  and  constitutes  the 
decidua  vera. 

As  the  ovum  passes  from  the  Fallopian  tube  into  the  interior  of 
the  uterus,  the  primitive  vitelline  membrane,  covered  with  villosities, 
becomes  entangled  with  the  processes  of  the  mucous  membrane.  A 
portion  of  the  decidua  vera  then  grows  up  on  all  sides  and  incloses 
the  ovum,  forming  the  decidua  reflexa,  while  the  villous  processes 
of  the  chorion  insert  themselves  into  the  uterine  tubules  and  in  the 
mucous  membrane  between  them. 

As  development  advances,  the  decidua  reflexa  increases  in  size, 
and  at  about  the  end  of  the  fourth  month  comes  in  contact  with  the 
decidua  vera,  with  which  it  is  ultimately  fused. 

The  Placenta. — Of  all  the  embryonic  structures,  the  placenta  is 
the  most  important.  It  is  formed  in  the  third  month,  and  then 


270  HUMAN   PHYSIOLOGY. 

increases  in  size  until  the  seventh  month,  when  a  retrogressive 
metamorphosis  takes  place  until  its  separation  during  labor,  at  which 
time  it  is  of  an  oval  or  rounded  shape,  and  measures  from  seven 
to  nine  inches  in  length,  six  to  eight  inches  in  breadth,  and  weighs 
from  fifteen  to  twenty  ounces.  It  is  most  frequently  situated  at 
the  upper  and  posterior  part  of  the  inner  surface  of  the  uterus. 

The  placenta  consists  of  two  portions,  a  fetal  and  a  maternal. 

The  fetal  portion  is  formed  by  the  villi  of  the  chorion,  which, 
by  developing,  rapidly  increase  in  size  and  number.  They  become 
branched  and  penetrate  the  \uterine  tubules,  which  enlarge  and  re- 
ceive their  many  ramifications.  The  capillary  blood-vessels  in  the 
anterior  of  the  villi  also  enlarge  and  freely  anastomose  with  one 
another. 

The  maternal  portion  is  formed  from  that  part  of  the  hypertrophied 
and  vascular  decidual  membrane  between  the  ovum  and  the  uterus, 
the  decidua  serotina.  As  the  placenta  increases  in  size,  the  maternal 
blood-vessels  around  the  tubules  become  more  and  more  numerous, 
and  gradually  fuse  together,  forming  great  lakes,  which  constitute 
sinuses  in  the  walls  of  the  uterus. 

As  the  terminal  period  of  gestation  approaches,  the  villi  extend 
deeper  into  the  decidua,  while  the  sinuses  in  the  maternal  portion 
become  larger  and  extend  further  into  the  chorion.  Finally,  from  ex*- 
cessive  development  of  the  blood-vessels,  the  structures  between 
them  disappear,  and  as  their  walls  come  in  contact  they  fuse  to- 
gether, so  that,  ultimately,  the  maternal  and  fetal  blood  are  separated 
only  by  a  thin  layer  of  a  homogeneous  substance.  When  fully 
formed,  the  placenta  consists  principally  of  blood-vessels  interlacing 
in  every  direction.  The  blood  of  the  mother  passes  from  the  uterine 
vessels  into  the  lake  surrounding  the  villi ;  the  blood  from  the  fetus 
flows  from  the  umbilical  arteries  into  the  interior  of  the  villi ;  but 
there  is  not  at  any  time  an  intermingling  of  blood,  the  two  being 
separated  by  a  delicate  membrane  formed  by  a  fusion  of  the  walls 
of  the  blood-vessels  and  the  walls  of  the  villi  and  uterine  sinuses. 

The  function  of  the  placenta,  besides  nutrition,  is  that  of  a 
respiratory  organ,  permitting  the  oxygen  of  the  maternal  blood  to 
pass  by  osmosis  through  the  delicate  placental  membrane  into  the 
blood  of  the  fetus  ;  at  the  same  time  permitting^  the  carbonic  acid 
and  other  waste  products,  the  result  of  nutritive  changes  in  the 
fetus,  to  pass  into  the  maternal  blood,  and  so  to  be  carried  to  the 
various  eliminating  organs. 


EMBRYOLOGY.  271 

Through  the  placenta  also  passes  all  the  nutritious  materials  of 
the  maternal  blood  which  are  essential  to  the  development  of  the 
embryo. 

At  about  the  middle  of  gestation  there  develops  beneath  the 
decidual  membrane  a  new  mucous  membrane,  destined  to  perform 
the  functions  of  the  old  when  it  is  extruded  from  the  womb,  along 
with  the  other  embryonic  structures,  during  parturition. 

DEVELOPMENT    OF    THE    EMBRYO. 

Nervous  System. — The  cerebro-spinal  axis  is  formed  within  the 
medullary  canal  by  the  development  of  cells  from  its  inner  sur- 
faces, which,  as  they  increase,  fill  up  the  canal,  and  there  remains 
only  the  central  canal  of  the  cord.  The  external  surface  gives  rise 
to  the  dura  mater  and  pia  mater.  The  neural  canal  thus  formed  is 
a  tubular  membrane ;  it  terminates  posteriorly  in  an  oval  dilatation, 
and  anteriorly  in  a  bulbous  extremity,  which  soon  becomes  partially 
contracted,  and  forms  the  anterior,  middle,  and  posterior,  cerebral 
vesicles,  from  which  are  ultimately  developed  the  cerebrum,  the 
corpora  quadrigemina,  and  the  medulla  oblongata,  respectively. 

The  anterior  vesicle  soon  subdivides  into  two  secondary  vesicles, 
the  larger  of  which  becomes  the  hemispheres,  the  smaller  the  optic 
thalami ;  the  posterior  vesicle  also  divides  into  two,  the  anterior 
becoming  the  cerebellum,  the  posterior  the  pons  Varolii  and  medulla 
oblongata. 

About  the  seventh  week  the  straight  chain  of  cerebral  vesicles 
becomes  curved  from  behind  forward  and  forms  three  prominent 
angles.  As  development  advances,  the  relative  size  of  the  encephalic 
masses  changes.  The  cerebrum,  developing  more  rapidly  than  the 
posterior  portion  of  the  brain,  soon  grows  backward  and  arches  over 
the  optic  thalami  and  the  tubercula  quadrigemina;  the  cerebellum 
overlaps  the  medulla  oblongata. 

The  surface  of  the  cerebral  hemispheres  is  at  first  smooth,  but  at 
about  the  fourth  month  begins  to  be  marked  by  the  future  fissures 
and  convolutions. 

The  eye  is  formed  by  a  little  bud  projecting  from  the  side  of  the 
anterior  vesicle.  It  is  at  first  hollow,  but  becomes  lined  with  nervous 
matter,  forming  the  optic  nerve  and  retina;  the  remainder  of  the 
cavity  is  occupied  by  the  vitreous  body.  The  anterior  portion  of  the 
pouch  becomes  invaginated  and  receives  the  crystalline  lens,  which 


272 


HUMAN    PHYSIOLOGY. 


is  a  product  of  the  epiblast,  as  is  also  the  cornea.  The  iris  appears 
as  a  circular  membrane  without  a  central  aperture,  about  the  seventh 
week ;  the  eyelids  are  formed  between  the  second  and  third  months. 

The  internal  ear  is  developed  from  the  auditory  vesicle,  budding 
from  the  third  cerebral  vesicle ;  the  membranous  vestibule  appears 
first,  and  from  it  diverticula  are  given  off,  which  become  the  semi- 
circular canals  and  the  cochlea. 

The  cavity  of  the  tympanum,  the  Eustachian  tube,  and  the  external 
auditory  canal  are  the  remains  of  the  first  branchial  cleft,  the  cavity 
of  this  cleft  being  subdivided  into  the  tympanum  and  external  audi- 
tory meatus  by  the  membrana  tympani. 

The  Skeleton. — The  chorda  dorsalis,  the  primitive  part  of  the 
vertebral  column,  is  a  cartilaginous  rod  situated  beneath  the  medul- 
lary groove.  It  is  a  temporary  structure,  and  disappears  as  the  true 
bony  vertebrae  develop.  On  either  side  are  the  quadrate  masses  of 
the  mesoblast,  the  primitive  vertebrae,  which  send  processes  upward 
and  around  the  medullary  groove,  and  downward  and  around  the 
chorda  dorsalis,  forming  in  these  situations  the  arches  and  bodies 
of  the  future  vertebrae. 

More  externally  the  outer  layers  of  the  mesoblast  and  epiblasl 
arch  downward  and  forward,  forming  the  ventral  laminae,  in  which 
develop  the  muscles  and  bones  of  the  abdominal  walls. 

The  true  cranium  is  an  anterior  development  of  the  vertebral 
column,  and  consists  of  the  occipital,  parietal,  and  frontal  segments, 
which  correspond  to  the  three  cerebral  vesicles.  The  base  of  the 
cranium  consists,  at  this  period,  of  a  cartilaginous  rod  on  either  side 
of  the  anterior  extremity  of  the  chorda  dorsalis,  in  which  three 
centers  of  ossification. appear,  the  basi-occipital,  the  basisphenoid,  and 
the  presphenoid.  They  ultimately  develop  into  the  basilar  process  of 
the  occipital  bone  and  the  body  of  the  sphenoid. 

The  entire  skeleton  is  at  first  either  membranous  or  cartilaginous. 
At  the  beginning  of  the  second  month  centers  of  ossification  appear 
in  the  jaws  and  clavicle;  as  development  advances  the  ossific  points  in 
all  the  future  bones  extend,  until  ossification  is  completed. 

The  limbs  develop,  from  four  little  buds  projecting  from  the  sides 
of  the  embryo,  which,  as  they  increase  in  length,  separate  into  the 
thigh,  leg,  and  foot,  and  the  arm,  forearm,  and  hand  ;  the  extremities 
of  the  limbs  undergo  subdivision,  to  form  the  fingers  and  toes. 


EMBRYOLOGY.  273 

Face  and  Visceral  Arches. — In  the  facial  and  cervical  regions 
the  visceral  laminae  send  up  three  processes,  the  visceral  arches, 
separated  by  clefts,  the  visceral  clefts. 

The  -first,  or  the  mandibular  arches,  unite  in  the  median  line  to 
form  the  lower  jaw,  and  superiorly  form  the  malleus.  A  process 
jutting"  from  its  base  grows  forward,  unites  with  the  frontonasal 
process  growing  from  above,  and  forms  the  upper  jaw.  When  the 
superior  maxillary  processes  fail  to  unite  there  results  the  cleft- 
palate  deformity ;  if  the  integument  also  fails  to  unite  there  results 
the  hare-lip  deformity.  The  space  above  the  mandibular  arch  becomes 
the  mouth. 

The  second  arch  develops  the  incus  and  stapes  bones,  the  styloid 
process  and  ligament,  and  the  lesser  cornu  of  the  hyoid  bone.  The 
cleft  between  the  first  and  second  arches  partially  closes  up,  but 
there  remains  an  opening  at  the  side,  which  becomes  the  Eustachian 
tube,  tympanic  cavity,  and  external  auditory  meatus. 

The  third  arch  develops  the  body  and  greater  cornu  of  the  hyoid 
bone. 

Alimentary  Canal  and  Its  Appendages. — The  alimentary  canal  is 
formed  by  a  pinching-off  of  the  yolk-sac  by  the  visceral  plates  as  they 
grow  downward  and  forward.  It  consists  of  three  distinct  portions 
— the  fore  gut,  the  hind  gut,  and  the  central  part,  which  communi- 
cates for  some  time  with  the  yolk-sac.  It  is  at  first  a  straight  tube, 
closed  at  both  extremities,  lying  just  beneath  the  vertebral  column. 
The  canal  gradually  increases  in  length  and  becomes  more  or  less 
convoluted ;  at  its  anterior  portion  two  pouches  appear,  which  be- 
come the  cardiac  and  pyloric  extremities  of  the  stomach.  At  about 
the  seventh  week  the  inferior  extremity  of  the  intestine  is  brought 
into  communication  with  the  exterior  by  an  opening,  the  anus.  An- 
teriorly the  mouth  and  pharynx  are  formed  by  an  involution  of  epi- 
blast,  which  deepens  until  it  communicates  with  the  fore  gut. 

The  liver  appears  as  a  slight  protrusion  from  the  sides  of  the  ali- 
mentary canal,  about  the  end  of  the  first  month  ;  it  grows  very  rapidly, 
attains  a  large  size,  and  almost  fills  up  the  abdominal  cavity.  The 
hepatic  cells  are  derived  from  the  intestinal  epithelium,  the  vessels 
and  connective  tissue  from  the  mesoblast. 

The  pancreas   is   formed   by   the   hypoblastic   membrane.      It   origi- 
nates in  two  small  ducts  budding  from  the  duodenum,  which  divide 
and  subdivide,  and  develop  the  glandular  structure. 
18 


274  HUMAN    PHYSIOLOGY. 

The  lungs  are  developed  from  the  anterior  part  of  the  esophagus. 
At  first  a  small  bud  appears,  which,  as  it  lengthens,  divides  into  two 
branches ;  second  and  tertiary  processes  are  given  off  from  these, 
which  form  the  bronchial  tubes  and  air-cells.  The  lungs  originally  ex- 
tended into  the  abdominal  cavity,  but  became  confined  to  the  thorax 
by  the  development  of  the  diaphragm. 

The  bladder  is  formed  by  a  dilatation  of  that  portion  of  the 
allantois  remaining  within  the  abdominal  cavity.  It  is  at  first  pear- 
shaped  and  communicates  with  the  intestine,  but  later  becomes  sep- 
arated and  opens  exteriorly  by  the  urethra.  It  is  attached  to  the 
abdominal  walls  by  a  rounded  cord — the  urachus,  the  remains  of  a 
portion  of  the  allantois. 

Genito-urinary  Apparatus. — The  Wolffian  bodies  appear  about  the 
thirteenth  day,  as  long,  hollow  tubes  running  along  each  side  of  the 
primitive  vertebral  column.  They  are  temporary  structures,  and  are 
sometimes  called  the  primordial  kidneys.  The  Wolffian  bodies  con- 
sist of  tubules  which  run  transversely  and  are  lined  with  epithelium ; 
internally  they  become  invaginated  to  receive  tufts  of  blood-vessels  ; 
externally  they  open  into  a  common  excretory  duct,  the  duct  of  the 
Wolffian  body,  which  unites  with  the  duct  of  the  opposite  body  and 
empties  into  the  intestinal  canal  at  a  point  opposite  the  allantois. 
On  the  outer  side  of  the  Wolffian  body  there  appears  another  duct, 
the  duct  of  Miiller,  which  also  opens  into  the  intestine. 

Behind  the  Wolffian  bodies  are  developed  the  structures  which 
become  either  the  ovaries  or  testicles.  In  the  development  of  the 
female  the  Wolffian  bodies  and  their  ducts  disappear ;  the  ex- 
tremities of  the  Miillerian  ducts  dilate  and  form  the  fimbriated 
extremity  of  the  Fallopian  tubes,  while  the  lower  portions  coalesce 
to  form  the  body  of  the  uterus  and  vagina,  which  now  separate  them- 
selves from  the  intestine. 

In  the  development  of  the  male  the  Miillerian  ducts  atrophy,  and 
the  ducts  of  the  Wolffian  body  ultimately  form  the  epididymis  and 
vas  deferens.  About  the  seventh  month  the  testicles  begin  to  descend, 
and  by  the  ninth  month  have  passed  through  the  abdominal  ring  into 
the  scrotum. 

The  kidneys  are  developed  out  of  the  Wolffian  bodies.  They  con- 
sist of  little  pyramidal  lobules,  composed  of  tubules  which  open  at 
the  apex  into  the  pelvis.  As  they  pass  outward  they  become  con- 
voluted and  cup-shaped  at  their  extremities,  receive  a  tuft  of  blood- 
vessels, and  form  the  Malpighian  bodies, 


EMBRYOLOGY.  275 

The  ureters  are  developed  from  the  kidneys  and  pass  downward 
to  be  connected  with  the  bladder. 

The  circulatory  apparatus  assumes  three  forms  at  different  periods 
of  life,  all  having  reference  to  the  manner  in  which  the  embryo 
receives  nutritive  matter  and  is  freed  of  waste  products. 

The  vitelline  circulation  appears  first  and  absorbs  nutritive  ma- 
terial from  the  vitellus.  It  is  formed  by  blood-vessels  which  emerge 
from  the  body  and  ramify  over  a  portion  of  the  vitelline  membrane, 
constituting  the  area  vasculosa.  The  heart,  lying  in  the  median 
line,  gives  off  two  arches,  which  unite  to  form  the  abdominal  aorta, 
from  which  two  large  arteries  are  given  off,  passing  into  the  vas- 
cular area ;  the  venous  blood  is  returned  by  veins  which  enter  the 
heart.  These  vessels  are  known  as  the  omphalo-mes enteric  arteries 
and  veins.  The  vitelline  circulation  is  of  short  duration  in  mam- 
mals, as  the  supply  of  nutritive  matter  in  the  vitellus  soon  becomes 
exhausted. 

The  placental  circulation  becomes  established  when  the  blood- 
vessels in  the  allantois  enter  the  villous  processes  of  the  chorion  and 
come  into  close  relationship  with  the  maternal  blood-vessels.  The 
circulation  lasts  during  the  whole  of  intra-uterine  life,  but  gives 
way  at  birth  to  the  adult  circulation,  the  change  being  made  possible 
by  the  development  of  the  circulatory  apparatus. 

The  heart  appears  as  a  mass  of  cells  coming  off  from  the  anterior 
portion  of  the  intestine ;  its  central  part  liquefies,  and  pulsations 
soon  begin.  The  heart  is  at  first  tubular,  receiving  posteriorly  the 
venous  trunks  and  giving  off  anteriorly  the  arterial  trunks.  It  soon 
becomes  twisted  upon  itself,  so  that  the  two  extremities  lie  upon 
the  same  plane. 

The  heart  now  consists  of  a  single  auricle  and  a  single  ventricle. 
A  septum,  growing  from  the  apex  of  the  ventricle,  divides  into 
two  cavities,  a  right  and  a  left.  The  auricles  also  become  partly 
separated  by  a  septum,  which  is  perforated  by  the  foramen  ovale. 
The  arterial  trunk  becomes  separated,  by  a  partition,  into  two 
canals,  which  become,  ultimately,  the  aorta  and  the  pulmonary 
artery.  The  auricles  are  separated  from  the  ventricles  by  incom- 
plete septa,  through  which  the  blood  passes  into  the  ventricles. 

Arteries. — The  aorta  arises  from  the  cephalic  extremity  of  the 
heart  and  divides  into  two  branches,  which  ascend,  one  on  each  side 
of  the  intestine,  and  unite  posteriorly  to  form  the  main  aorta ;  pos- 


276  HUMAN    PHYSIOLOGY. 

teriorly  to  these  first  aortic  arches  four  others  are  developed,  so 
that  there  are  five  altogether  running  along  the  visceral  arches.  The 
two  anterior  soon  disappear.  The  third  arch  becomes  the  internal 
carotid  and  the  external  carotid;  a  part  of  the  fourth  arch,  on  the 
right  side,  becomes  the  subclavian  artery,  and  the  remainder  atrophies 
and  disappears,  but  on  the  left  side  it  enlarges  and  becomes  the 
permanent  aorta ;  the  fifth  arch  becomes  the  pulmonary  artery  on  the 
left  side.  The  communication  between  the  pulmonary  artery  and  the 
aorta,  the  ductus  arteriosus,  disappears  at  an  early  period. 

Veins. — The  venous  system  appears  first  as  two  short,  transverse 
veins,  the  canals  of  Cuvier,  formed  by  the  union  of  the  vertebral 
veins  and  the  cardinal  veins,  which  empty  into  the  auricle.  The 
inferior  vena  cava  is  formed,  as  the  kidneys  develop,  by  the  union 
of  the  renal  veins,  which,  in  a  short  time,  receive  branches  from 
the  lower  extremities.  The  subclavian  veins  join  the  jugular  as  the 
upper  extremities  develop.  The  heart  descends  in  the  thorax,  and 
the  canals  of  Cuvier  become  oblique  ;  they  shortly  communicate  by 
a  transverse  duct,  which  ultimately  becomes  the  left  innominate 
vein.  The  left  canal  of  Cuvier  atrophies  and  becomes  a  fibrous  cord. 
A  transverse  branch  now  appears,  which  carries  the  blood  from  the 
left  cardiac  vein  into  the  right,  and  becomes  the  vena  azygos  minor ; 
the  right  cardiac  vein  becomes  the  vena  azygos  major. 

Circulation  of  Blood  in  the  Fetus. — The  blood  returning  from  the 
placenta,  after  having  received  oxygen  and  being  freed  from  car- 
bonic acid,  is  carried  by  the  umbilical  vein  to  the  under  surface  of 
the  liver ;  here  a  portion  of  it  passes  through  the  ductus  venosus  into 
the  ascending  vena  cava,  while  the  remainder  flows  through  the 
liver  and  passes  into  the  vena  cava  by  the  hepatic  veins.  When  the 
blood  is  emptied  into  the  right  auricle,  it  is  directed  by  the  Eustachian 
valve  through  the  foramen  ovale,  into  the  left  auricle,  thence  into 
the  left  ventricle,  and  so  into  the  aorta  and  to  all  parts  of  the 
system.  The  venous  blood  returning  from  the  head  and  upper  ex- 
tremities is  emptied,  by  the  superior  vena  cava,  into  the  right 
auricle,  from  which  it  passes  into  the  right  ventricle,  and  thence 
into  the  pulmonary  artery.  Owing  to  the  condition  of  the  lung 
only  a  small  portion  flows  through  the  pulmonary  capillaries,  the 
greater  part  passing  through  the  ductus  arteriosus,  which  opens  into 
the  aorta  at  a  point  below  the  origin  of  the  carotid  and  subclavian 
arteries.  The  mixed  blood  now  passes  down  the  aorta  to  supply  the 


EMBRYOLOGY.  277 

lower  extremities,  but  a  portion  of  it  is  directed,  by  the  hypogastric 
arteries,  to  the  placenta,  to  be  again  oxygenated. 

At  birth,  the  placental  circulation  gives  way  to  the  circulation  of 
the  adult.  As  soon  as  the  child  begins  to  breathe,  the  lungs  expand, 
blood  flows  freely  through  the  pulmonary  capillaries,  and  the  ductus 
arteriosus  begins  to  contract.  The  foramen  ovale  closes  about  the 
tenth  day.  The  umbilical  vein,  the  ductus  venosus,  and  the  hypo- 
gastric  arteries  become  impervious  in  several  days,  and  ultimately 
form  rounded  cords. 


TABLE    OF    PHYSIOLOGICAL    CONSTANTS. 


Mean  height  of  male,  5  feet  6^  inches;  of  female,  5  feet  2  inches. 
Mean  weight  of  male,  145  pounds;  of  female,  121  pounds. 
Number  of  chemic  elements  in  the  human  body:  from  16  to  18. 
Number  of  proximate  principles  in  the  human  body:  about  100. 
Amount  of  water  in  the  body  weighing  145  pounds:  109  pounds. 
Amount  of  solids  in  the  body  weighing  145  pounds:  36  pounds. 
Amount  of  saliva  secreted  in  24  hours:  about  3^  pounds. 
Function  of  saliva:    converts  starch  into  maltose. 
Active  principle  of  saliva:  ptyalin. 

Amount  of  gastric  juice  secreted  in  24  hours:  from  8  to  14  pounds. 
Function  of  gastric  juice:   converts  albumin  into  peptone. 
Active  principles  of  gastric  juice:  pepsin  and  hydrochloric  acid. 
Duration  of  digestion:  from  3  to  5  hours. 
Amount  of  intestinal  juice  secreted  in  24  hours :  about  i  pound. 

Function  of  intestinal  juice:   converts  cane  sugar  into  dextrose 

and  levulose ;  maltose  into  dextrose. 

Amount  of  pancreatic  juice  secreted  in  24  hours :  about  i  y*  pounds. 
Active  principles  of  pancreatic  juice:  trypsin,  amylopsin,  and 
steapsin. 

.  Steapsin  splits  the  neutral  fats  into  fatty  acids  and 

glycerin. 

1     2.  Trypsin  converts  albumin  into  peptone. 
3.  Amylopsin  converts  starch  into  maltose. 
Amount  of  bile  poured  into  the  intestines  daily:  about  2^  pounds. 

1.  Assists  in  the  emulsification  of  fats. 

2.  Stimulates   the  peristaltic   movements. 
Functions  :J         _  ....  .      -      ,.      , 

3.  Prevents  putrefactive  changes  in  the  food. 

4.  Promotes   the   absorption   of   fat. 

Amount  of  blood  in  the  body :  about  -^  of  the  body  weight. 
Size  of  red  corpuscles :  -^V  o~  of  an  *ncn- 
Size  of  white  corpuscles :  -%•%-$•$  of  an  inch. 
Shape  of  red  corpuscles:  circular  biconcave  discs. 
Shape  of  white  corpuscles:   globular. 

Number  of  red  corpuscles  in  a  cubic  millimeter  of  blood  (the  cubic 
•^-5-  of  an  inch)  :  5,000,000. 

278 


TABLE  OF  PHYSIOLOGICAL  CONTENTS.  .  279 

Function  of  red  corpuscles:   to  carry  oxygen  from  the  lungs  to  the 

tissues. 

Frequency  of  the  heart's  pulsation  a  minute :   72  on  the  average. 
Velocity  of  the  blood  movement  in  the  arteries:  about  12  inches  a 

second. 
Length  of  time  required  for  the  blood  to  make  an  entire  circuit  of 

the  vascular  system  :  about  20  seconds. 
Amount  of  air  passing  in  and  out  of  the  lungs  at  each  respiratory 

act :  from  20  to  30  cubic  inches. 

Amount   of  air  that   can   be   taken   into   the   lungs   on   a   forced   in- 
spiration:  no  cubic  inches. 
Amount  of  reserve  air   in  the   lungs   after   an   ordinary  expiration ; 

100  cubic  inches. 
Amount  of  residual  air  always  remaining  in  the  lungs:   about   100 

cubic  inches. 

Vital  capacity  of  the  lungs:   230  cubic  inches. 
Entire  volume  of  air  passing  in  and  out  of  the  lungs  in  24  hours : 

about  400  cubic  feet. 

Composition  of  the  air:  nitrogen,  79.19;  oxygen,  20.881,  in  100  parts. 
Amount  of  oxygen  absorbed  in  24  hours:  18  cubic  feet. 
Temperature  of  the  human  body  at  the  surface:  98^°  F. 
Amount  of  urine  excreted  daily :  from  40  to  50  ounces. 
Amount  of  urea  excreted  daily:   512  grains. 
Specific  gravity  of  urine:  from  1015  to  1025. 
Number  of  spinal  nerves:   31  pairs. 
Number  of  roots  of  origin:  two;  ist,  anterior,  efferent;  2d,  posterior, 

afferent. 

Rate  of  transmission  of  nerve  force:  about  100  feet  a  second. 
Number  of  cranial  nerves:  12  pairs. 

1.  Olfactory,  or  first  pair. 

2.  Optic,   or  second  pair. 

3.  Auditory,  or  eighth  pair. 

4.  Chorda  tympani  for  anterior  two  thirds 
Nerves  of  special  sense:^         of  tongue. 

5.  Branches      of     glosso-pharyngeal,      or 
eighth  pair,   for  posterior  one  third  of 
tongue. 

Motor  nerves  to  eyeball  and  accessory  structures:   motor  oculi,  or 
third  pair ;  pathetic,  or  fourth  pair ;  abducens,  or  sixth  pair. 


^80  HUMAN    PHYSIOLOGY. 

Motor  nerve  to  facial  muscles:  portio  dura,  facial,  or  seventh  pair. 

Motor  nerve  to  tongue:   hypoglossal,  or  twelfth  pair. 

Motor  nerve  to  laryngeal  muscles:  spinal  accessory,  or  eleventh  pair. 

Sensory  nerve  of  the  face:   trigeminal,  or  fifth  pair. 

Sensory  nerves  of  the  pharynx:   glosso-pharyngeal,  or  ninth  pair. 

Sensory  nerves  of  the  lungs,  stomach,  etc.:  pneumogastric,  or  tenth 

pair. 

Length  of  spinal  cord:   16  to  18  inches;  weight,  il/2  ounces. 
Point  of  decussation  of  motor  fibers:   at  the  medulla  oblongata. 
Point  of  decussation  of  sensory  fibers:  throughout  the  spinal  cord. 
Function  of  anterolateral  column  of  spinal  cord:   transmit  motor 

impulses  from  the  brain  to  the  muscles. 
Function   of   the   posterior   columns:    assist   in   the   coordination   of 

muscular  movements. 
Function   of  the  medulla   oblongata:    controls  the   functions   of  in- 

salivation,  mastication,  deglutition,  respiration,  circulation,  etc. 
Function  of  the  cerebellum:   center  for  the  coordination  of  muscular 

movement. 

Function  of  the  cerebrum:   center  for  intelligence,  reason,  and  will. 
Center  for  articulate  language:  third  frontal  convolution  on  the  left 

side  of  cerebrum. 
Number  of  coats  to   the  eye:   three:    ist,   cornea   and   sclerotic;   2d, 

choroid  ;  3d,  retina. 

Function  of  iris:  regulates  the  amount  of  light  entering  the  eye. 
Function  of  crystalline  lens:  refracts  the  rays  of  light  so  as  to  form 

an  image  on  the  retina. 

Function  of  retina:   receives  the  impressions  of  light. 
Function  of  membrana  tympani:    receives  and  transmits  waves  of 

sound  to  internal  ear. 
Function  of  Eustachian  tube:  regulates  the  passage  of  air  into  and 

from  the  middle  ear. 
Function  of  semicircular  canals:   assist  in  maintaining  the  equipoise 

of  the  body. 
Function  of  the  cochlea:   appreciates  the  shades  and  combinations  of 

musical  tones. 

Size  of  human  ovum:  T|^  of  an  inch  in  diameter. 
Size  of  spermatozoa:  ¥UL^  of  an  inch  in  length. 
Function  of  the  placenta:   acts  as  a  respiratory  and  digestive  organ 

for  the  fetus. 
Duration  of  pregnancy:   280  days. 


TABLE  SHOWING  RELATION  OF  WEIGHTS  AND  MEASURES 
OF  THE  METRIC  SYSTEM  TO  APPROXIMATE  WEIGHTS 
AND  MEASURES  OF  THE  UNITED  STATES. 


MEASURES   OF  LENGTH. 


One  Myriameter 

=     10,000  meters                        =• 

32,800       feet. 

One  Kilometer 

=•           I,OOO         "                                            — 

3,280          " 

One  Hectometer 

±5         .     ZOO.      "                                      = 

328          " 

One  Decameter 

=                  10        "                                          = 

32.80     " 

!the    ten-millionth    part  j 

One  METER 

of  a  quarter  of  the  Me-  1  = 

39.368            inches. 

ridian  of  the  Earth.        J 

One  Decimeter 

=     the  tenth  part  of  i  meter     = 

3.936 

One  Centimeter 

—  {  o^one  m^te^re    h  ^  }  = 

0.393(^5)    inch. 

One  Millimeter 

j  the  one-thousandth  part  1 
\  of  one  meter. 

0.039  (A)        " 

WEIGHTS. 

One  Myriagram 

=     1  0,000  grams                          = 

26^  pounds   Troy. 

One  Kilogram 

=        1,000     "                                = 

2%        " 

One  Hectogram 

=           100     "                                = 

3l/4  ounces       " 

One  Decagram 

—             10     "                                = 

zl/2  drams        " 

One  GRAM 

f  the  weight  of  a  cubic  cen-  1  
~  \  timeter  of  water  at  4°  C.  J  ~ 

15.434            grains. 

One  Decigram 

=     the  tenth  part  of  a  gram     = 

1.543  (iH)       "' 

One  Centigram 

=     the  looth  part  of  i  gram     = 

0.154  (J^)    grain. 

One  Milligram 

f  the  thousandth  part  of  )  

0-015  (^) 

\  one  gram                            j 

MEASURES    OF    CAPACITY 

{10  cubic  Meters  or  the") 

One  Myrialiter 

measures  of  10  Milliers  >  = 

2,600     gallons. 

of  water                              J 

C  i    cubic    Meter    or    the  ) 

One  Kiloliter 

—  -j  measure    of    i     Millier  >•  = 

260 

1  of  water                             ) 

One  Hectoliter 

f  100     cubic    Decimeters") 
=  •!  or    the    measure    of    i  >•  = 
(  Quintal  of  water              J 

26 

C  10  cubic  Decimeters  or") 

One  Decaliter 

=  •!  the  measure  of  i  Myri-  >  = 

2.6          " 

{  agram  of  water                J 

!i    cubic    Decimeter    or  | 

One  LITER 

the  measure  of  i   Kilo-  >•  = 

2.1  pints. 

gram  of  water                   J 

One  Deciliter 

C  100    cubic    Centimeters  1 
—  -j  or    the    measure    of    t  f  •.= 
1  Hectogram    of   water     J 

3.3  ounces. 

f  10  cubic  Centimeters  or) 

One  Centiliter 

=  -j  the  measure  of  i  Deca-  \  = 

2.7  drams. 

(  gram  of  water 

f  i    cubic    Centimeter    or  ^ 

One  Milliliter 

=  -I  the  measure  of  i  Gram  >•  = 

16.2  minims. 

(  of  water                             J 

INDEX. 


ABDUCENS    NERVE    ......   222 

<£•»•     Aberration,  chromatic   ----   246 

spheric  ..............   246 

Absorption  ...................    114 

by    lacteals    ..............    119 

by  blood-vessels   ..........    119 

of  oxygen  in  respiration   .  .    149 
Accommodation   of   the   eye    .  .  .   245 
Adipose   tissue,    uses  of,   in  the 
body    ......................     38 

Adrenal  bodies  ...............    162 

Adult   circulation,    establishment 
of,  at  birth   .....  ...........   261 

Air,  atmospheric,  composition  of.   148 
amount  exchanged  in  respi- 
ration   .................    148 

changes   in,    during   respira- 
tion   ...................    148 

Albumin,   uses  of,  in  the  body.     89 
Albuminoids    .................     24 

Alcohol,  action  of   ............      91 

Alimentary    canal,    development 
of 


principles,  classification  of.. 
albuminous  principles  ..... 
saccharine  principles  ...... 


258 
88 
88 
89 
89 
89 


oleaginous  principles  ...... 

inorganic  principles  ....... 

Allantois,  development  and  func- 

tion of    ....................  268 

Amnion,  formation  of  .........  268 

Animal  heat    .................  156 

Anterior  columns  of  spinal  cord.  182 

Area,  germinal  ...............  252 

Areolar  tissue  ................  37 

Arteries,    properties   of    .......  137 

Articulations     ................  44 

classification   of    ..........  44 

Asphyxia    ....................  150 

Astigmatism  ..................  246 


"RILE    112 

'     Bladder,  urinary 167 

Blastodermic  membranes 252 

Blood    123 

composition  of,  plasma  ....  124 

coagulation   of    127 

coloring-matter  of    126 

changes  in,    during  respira- 
tion      149 

circulation   of    129 

rapidity  of  flow  in  arteries.  139 


PAGE 

Blood,  rapidity  of  flow  in  capilla- 
ries      140 

corpuscles    125 

origin  of    127 

pressure    138 

Bone,  structure  of 41 

Burdach,  column  of   183 

pANALS  OF  CUVIER   261 

^     Capillary  blood-vessels  ....  140 

Capsule,  internal   199 

external    199 

Carbohydrates 9 

Cartilage    39 

Caudate  nucleus 199 

Cells,  structure  of 29 

manifestation   of  life  by...  31 
of    anterior    horns    of    gray 

matter     182 

Center  for  articulate  language..  212 
Central  organs  of  the  nerve  sys- 
tem and  their  nerves 180 

Cerebellum    201 

forced    movements   of    ....  202 

Cerebral  vesicles  of  embryo  ....  256 

Cerebrum 202 

fissures   and    convolutions.  .  203 

functions  of    207 

localization  of   functions.  . .  210 

motor  area  of 211 

sensor   centers   of    214 

Chemic    composition    of    human 

body    8 

Chorda  dorsalis   257 

tympani    nerve,    course    and 

function  of    226 

Chorion 268 

Chyle 121 

Ciliary   muscle    245 

Circulation  of  blood   129 

Claustrum    199 

Cochlea     242 

Columns  of  spinal  cord 183 

Connective     tissues,     physiologic 

properties  of    37,  42 

Corium 166 

Corpora  Wolffiana 259 

quadrigemina    198 

Corpus  luteum    268 

striatum    199 

Corti,  organ  of 243 

Cranial  nerves   218 


283 


284 


Crura  cerebri   197 

Crystalline   lens    242 

•pvECIDUAL  MEMBRANE   ..  269 

*-*     Deglutition     101 

Development  of  accessory  struc- 
tures of  embryo 266 

Digestion   96 

Ductus   arteriosus    276 

venosus 276 

EAR    249 
Electrotonus    85 

Embryo,    development   of    271 

Endolymph    257 

Epididymis     264 

Eustachian  tube 251,  254 

Excretion 164 

Eye    237 

refracting  apparatus  of  ....   243 
blind  spot  of 247 

•RACIAL  NERVE  224 

paralysis,     symptoms    of.  225 

Fallopian   tubes    262 

Fat,  uses  of,  in  the  body 38 

Female  organs  of  generation   .  .  261 
Fissures     and     convolutions     of 

brain   203 

Foods  and  dietetics 86 

animal  95 

vegetable    95 

cereal    96 

percentage  composition  of  .95,  96 

daily  amount  required    ....  93 

albuminous  principles  of...  88 

saccharine  principles  of  ...  89 

oleaginous  principles  of  ...  89 

inorganic   principles   of    ...  89 

Fovea    centralis    247 

pALVANIC  CURRENTS,  EF- 

^J     feet   on   nerves    85 

Ganglia  215 

ophthalmic 215 

Gasserian    215 

spheno-palatine     215 

otic 215 

sub-maxillary 215 

semilunar 216 

Gastric  digestion    102 

juice    103 

action    of     104 

Generation,  male  organs  of  ....  264 

female  organs  of 261 

Globules  of  the  blood    125 

of  the  lymph    120 

Glomeruli  of  the  kidneys 166 

Glosso-pharyngeal    nerve    227 

Glottis,     respiratory     movements 

of     145 

Glycogen 175 

Glycogenic  function  of  the  liver.  175 

Goll,  column  of 184 

Graafian  follicles   261 


PAGE 

TLTAIR     178 

*-*-     Hearing,  sense  of 249 

Heart    129 

valves  of    130 

sounds  of 134 

influence    of    pneumogastric 

nerve  upon    137 

fanglia  of 136 
orce    exerted   by    left   ven- 
tricle     135 

work  done  by 135 

course  of  blood  through  ...  131 
influence  of  nervous  system 

upon    136 

Hemoglobin 126 

Hyaloid  membrane    242 

Hypermetropia    247 

Hypoglossal  nerve 232 

TNCUS  BONE   250 

•*•     Insalivation     98 

nerve  circle  of 100 

Inspiration,   movements  of  thor- 
ax  in 144 

Internal    capsule    199 

results  of  injury  to 199 

Intestinal   juice    109 

Iris '.  . .  247 

action  of    247 

Island  of  Reil    205 

T/-IDNEYS    164 

•*•*•         excretion  of  urine  by. .  .  165 

T  ABYRINTH      OF      INTER- 

A-'        nal  ear 255 

function  of  cochlea    257 

function       of "     semicircular 

canals   256 

Language,  articulate,  center  for.  212 

Larynx   244 

Lateral  columns  of  spinal  cord.  .  183 

Laws  of  muscular  contraction.  .  86 

Lens,    crystalline    242 

Levers,    62 

Lime    Phosphate    25 

Liver 172 

secretion   of  bile  by    174 

glycogenic  function  of   ....  175 

formation  of  urea   .  % 176 

Localization  of  functions  in  cere- 
brum       210 

Lungs   142 

changes  in  blood  while  pass- 
ing through 149 

Lymph    120 

Lymphatic  glands 117 

vessels,  origin  and  course  of.   115 

iwrALLEUS  BONE 237 

*VJ-     Mammary  glands i55 

Mastication    96 

nerve  mechanism  of 97 

muscles  of 97 


285 


Medulla  oblongata 193 

properties  and  functions  of.  194 

Membrana  tympani    237 

Menstruation 248 

Middle  ear 237 

Milk    156 

Motor  centers  of  cerebrum  ....  211 

Muscles,   properties  of    48 

changes  in,   during  contrac- 
tion   .  56 

special  physiology  of 61 

Muscle-fiber,  histology  of 50,  51 

Myopia 246 


ATERVE,  OLFACTORY   219 

*^    optic    219 

motor    oculi     220 

pathetic     221 

trigeminal     222 

abducens 222 

facial  224 

auditory    226 

glosso-pharyngeal    227 

pneumogastric 228 

spinal   accessory 230 

hypoglossal    232 

cells,  structure  of   69 

fibers,  structure  of 71 

terminations  of 75 

impulse,    rate    of    transmis- 
sion of   84 

roots,    function    of    anterior 

and   posterior    76 

tissue,  histology  of 68 

trunks,   structure   of 72 

Nerves,    centrifugal    and   centri- 
petal      77 

classification  of 74 

relation  of,  to  spinal  cord..  76 
development    and    nutrition 

of 77 

cranial    218 

vaso-motor    195 

properties  and  functions  of.  82 

spinal    173 

Nervous  tissue,   physiology  of .  .  68 

sympathetic    215 

Neuron    -70 

Nucleus  caudatus 199 

lenticularis 199 

OLFACTORY  NERVES    219 

Ophthalmic  ganglion    215 

Optic  nerves 219 

thalamus     '. 199 

functions  of 200 

Organs  of  Corti 243 

Osazones 14 

Otic   ganglion    215 

Ovaries  261 

Ovum    :  261 

discharge      of,       from      the 

ovary 262 

Oxygen,  absorption  of,  by  hemo- 
globin      126 


PAGE 

pANCREATIC  JUICE   no 

Patheticus  nerve    221 

Peptones    107 

Perilymph    242 

Perspiration     179 

Petrosal  nerves,  large  and  small.  225 

Phonation    246 

Physiology,   definition  of    i 

Placenta,    formation    and    func- 
tion of   269 

Pleura 143 

Pneumogastric   nerve    228 

Pons  varolii    197 

Portal  vein    1 1 8 

Posterior      columns     of      spinal 

cord     184 

functions  of    191 

Prehension 95 

Presbyopia     _ 247 

Pressure  of  blood  in  arteries.  . .  .  138 

Proximate  principles 9 

inorganic    24 

carbohydrates    9,  141 

proteids 15 

of  waste 28 

quantity      of      chemjc      ele- 
ments in  body 9 

Ptyalin 100 

Pulse     139 

Pyramidal  tracts    183 

ED       CORPUSCLES       O  F 

blood    125 

Reflex  movements  of  spinal  cord.  185 

action,    laws   of    186 

Reproduction     244 

Respiration    142 

movements  of 144 

types  of   146 

nerve  mechanism 145 

Retina     225 

Rigor  mortis 52 


99 
178 
154 
265; 
255 
237 

43 
177 
236 
134 
265 
215 


R 


o  ALIVA    

"^     Sebaceous  glands 
Secretion    

Semen 


Semicircular  canals  

Sight,  sense  of 

Skeleton 

Skin  

Smell,  sense  of  

Sounds  of  heart 

Spermatozoa  

Spheno-palatine  ganglion  

Spinal  accessory  nerve 

cord     

cord,  membranes  of    

structure  of  white  matter.  . 

structure  of  gray  matter.  .  .  . 

properties  of 

function  of,  as  a  conductor. 

as   an  independent  center.  . 

reflex  action  of   

special  centers   of    


183 
182 
191 
189 
185 


286 


Spinal  accessory  nerve,  paralysis 

from  injuries  of    192 

nerves,  origin  of   77 

Starvation,   phenomena   of    ....      87 
Stomach   102 


Submaxillary    ganglion    216 

Sudoriparous  glands 179 

Sugar,  uses  of,  in  the  body  ....  90 

Supra-renal  capsules 162 

Sympathetic  nervous  system  ...  215 

properties  and  functions  of.  217 

TASTE,  SENSE  OF 234 

•*•          nerve  of 235 

Teeth 97 

Tensor  tympani  muscle  ....250,  253 

Testicles 264 

Thoracic  duct 117 

Thorax,    enlargement   of,    in   in- 
spiration         144 

Tongue   234 

motor  nerve  of    235 

sensory  nerve  of   235 

Touch,  sense  of 233 

Tiirck,   column  of 183 


PAGE 

T  TMBILICAL  CORD 269 

U     Urea    170 

Uric    acid    170 

Urination,  nerve  mechanism  of.    167 

Urine    i58 

composition    of     169 

average  quantity  of  constitu- 
ents secreted  daily 169 

Uterus    262 

VAPOR,      WATERY,     OF 

breath    148 

Vascular  glands   158 

system,    development    of    .  ,  275 

Vaso-motor   nerves,    origin   of .  .  195 

Veins    140 

Vertebral   column    46 

Vesiculse   seminales    265 

Vision,   psychic   center   for    214 

Vital  capacity  of  lungs 147 

Vocal  cords 258 

Voice    259 

WATER,   AMOUNT   OF,    IN 
body     24 

Wolffian   bodies    274 


A  Textbook  of 
Human   Physiology 

By  A.   P.  BRUBAKER,   M.D. 

Professor  of  Physiology  and  Hygiene  at  Jefferson  Medical  College;  Pro- 
fessor of  Physiology,   Pennsylvania   College  of 
Dental  Surgery,    Philadelphia. 

Second  Edition,  Revised  and  Enlarged.  With  Colored  Plates  and  356 
other  Illustrations.  Octavo;  715  pages.  Cloth,  $4.00;  Leather  or  Half- 
Morocco,  $5.00,  net. 

The  object  in  view  in  the  preparation  of  this  volume  was  the  selection  and 
presentation  of  the  more  important  facts  of  physiology,  in  a  form  healthful  to 
students  and  to  practitioners  of  medicine.  Such  facts  have  been  selected  as 
will  not  only  elucidate  the  normal  functions  of  the  tissues  and  organs  of  the 
body,  but  which  will  be  of  assistance  in  understanding  their  abnormal  mani- 
festations as  they  present  themselves  in  hospital  and  private  work.  In  the 
method  of  presentation,  the  author  has  been  guided  by  an  experience  gained 
during  twenty  years  of  active  teaching. 

"  *  *  Will  rank  with  the  very  best  of  modern  treatises  on  physiology.  We 
unhesitatingly  recommend  it  to  physicians  and  students  as  an  accurate  and 
admirably  written  textbook  on  the  subject."— American  Medicine,  Philadelphia. 

SYNOPSIS  OF  CONTENTS: 

Introduction — Chemic  Composition  of  the  Human  Body — Physiology  of 
the  Cell — Histology  of  the  Epithelial  and  Connective  Tissues — The  Physiology 
of  the  Skeleton — General  Physiology  of  Muscle-Tissue — General  Physiology 
of  Nerve-Tissue — Foods — Digestion — Absorption — The  Blood — Circulation  of 
the  Blood  —  Respiration  —  Animal  Heat  —  Secretion  —  Excretion  —  Central 
Organs  of  the  Nerve  System  and  their  Nerves — The  Medulla  Oblongata  ;  the 
Isthmus  of  the  Encephalon  ;  the  Basal  Ganglia — The  Cerebrum — The  Cere- 
bellum— Cranial  Nerves — Sympathetic  Nerve  System — Phonation  ;  Articulate 
Speech — The  Special  Senses — Sense  of  Sight — Sense  of  Hearing — Reproduc- 
tion— Physiologic  Apparatus — Index . 

"  Prof.  Brubaker  is  known  to  most  students  in  the  United  States  through 
his  excellent  compendium  on  physiology.  The  present  volume  is  more  pre- 
tentious and  covers  the  field  of  physiology  as  applicable  to  everyday  practice. 
*  *  We  know  of  no  American  textbook  which  contains  as  much  sound  physi- 
ological information  as  that  of  Brubaker."— New  Orleans  Medical  and  Surgical 
fournal. 


Morris5   Anatomy 

Third  Edition,  Revised 
846  Illustrations 

A  Complete  Textbook.  Edited  by  HENRY  MORRIS,  F.R.C.S.,  Surgeon 
to,  and  Lecturer  on  Anatomy  at,  Middlesex  Hospital.  Assisted  by  PETER 
THOMPSON,  M.D.,  J.  BLAND  SUTTON,  F.R.C.S.,  J.  H.  DAVIES-COOLEY, 
F.R.C.S.,  WM.  J.  WALSHAM,  F.R.C.S.,  H.  ST.  JOHN  BROOKS,  M.D.,  R. 
MARCUS  GUNN,  F.R.C.S.,  ARTHUR  HENSMAN,  .  F.R.C.S.,  FREDERICK 
TREVES,  F.R.C.S.,  WILLIAM  ANDERSON,  F.R.C.S.,  ARTHUR  ROBINSON, 
M.D.,  M.R.C.S.,  and  Prof.  W.  H.  A.  JACOBSON. 

One  Handsome  Octavo  Volume,  with  846  Illustrations,  of  which  267  are 
printed  in  Colors.  Thumb  Index  and  Colored  Illustrations  in  all  Copies. 
Third  Revised  Edition,  Enlarged  and  Improved.  Octavo ;  1328  pages. 
Cloth,  $6.00;  Leather,  $7.00;  Half-Russia,  $8.00,  n  t. 

A  prominent  reviewer  writes  : 

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language,  *  Morris '  is  undoubtedly  the  most  up-to-date  and  accurate.     ^ 
For  the  student,  the  surgeon,  or  for  the  general  practitioner  who  desires  to  re- 
view his  anatomy,  '  Morris'  is  decidedly  the  book  to  buy." 

Morris'  Anatomy  is  now  the  recognized  standard  textbook  in  a  large  num- 
ber of  medical  schools  throughout  the  United  States,  England  and  Canada. 
The  Third  Edition  has  been  carefully  revised  and  improved  both  in  regard  to 
text  and  illustrations,  the  number  of  the  latter  having  been  increased  by  56,  of 
which  50  are  in  colors.  Many  of  those  in  the  former  editions  have  been  dis- 
placed by  new  drawings,  while  others  have  been  altered  in  important  details. 
Upwards  of  $1,000  has  been  spent  on  improving  this  feature  alone.  Two  sec- 
tions have  been  almost  entirely  rewritten,  every  effort  having  been  employed 
to  make  the  book  more  useful  to  students.  Many  of  the  features  that  formerly 
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have  had  special  attention,  and  less  important  matters,  but  none  the  less  con- 
fusing ones  to  the  student,  have  had  careful  consideration. 

"The  various  sections  have  been  written  by  authors  of  world-wide  fame, 
and  their  individual  labors  have  been  combined  into  a  most  harmonious 
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—  Chicago  Medical  Recorder. 

"  The  ever-growing  popularity  of  the  book  with  teachers  and  students  is 
an  index  of  its  value,  and  it  may  safely  be  recommended  to  all  interested."  — 
Medical  Record,  New  York. 

**  "  A  Guide  to  Dissection,"  based  upon  "  Morris,"  has  been  prepared  by 
Dr.  Simon  M.  Yutzy,  Demonstrator  of  Anatomy  at  the  University  of  Michi- 
gan, Ann  Arbor.  Price  25!Vents. 


cA    Companion    Volume    to    Gould's    Docket    ^Dictionary 

A  POCKET  CYCLOPEDIA 


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MEDICINE  ^  SURGERY 


EDITED  BY 

GEORGE  M.  GOVLD,  A.M.,  M.D. 

Author  of  "  Gould's  Medical  Dictionaries *  "  Editor  of  "  American  Medicine'' 
AND 

WALTER  L.  PYLE,  A.M.,  M.D. 

Assistant  Surgeon  Wills  Eye  Hospital,  Philadelphia  j  formerly  Editor 
"  International  Medical  Magazine,"  etc. 


BEINO  BASED  UPON  GOULD  AND  PYLE'S  LARGE  "  CYCLOPEDIA  OF 
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CASPER.  A  TEXT-BOOK  OF  GENITO-URINARY  DISEASES, 
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MAN.  By  LEOPOLD  CASPER,  M.D.,  Professor  in  the  University  of  Berlin. 
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